I think an angle that some are missing is that this the time to learn the knowledge and figure things out is in some ways a red herring. While I don't per se disagree with the accepted answer, I think it still misses something important.
This is people asking you to, in the present day, to currently do work for free for them, with a presumption that because the work involved is presumably "just" "imparting knowledge", it carries no burden.
That some people volunteer their time and skills to answer questions on a site like SE is also a red herring for situations where that work is being directly requested of someone, with a clear presumption of expecting it to simply be given for free. There are a number of contextual differences, but the primary one is the valuation of someone's individual time and skills and agency.
Rather than focusing on the more "proprietary knowledge" aspects where discussing this has an associated cost background for you (your time spent learning), I would instead be upfront that this is a complex topic and that the results rely on a lot of your own work and knowledge, not just things someone can go grab off a website and install somewhere and achieve the same result in readymade fashion.
Soft Opening wrapping Firm Boundaries
I would personally start off with mentioning the basic overview in generic terms, such as "it's a WordPress based site primarily driven with custom plugins and themes", because if you feel like you're giving something special away (regardless of field) with a cursory overview of the fundamental techniques/off the shelf tools used, I can happily tell you: you're not. Even if they seem to be asking for "specific" details (it's hard to tell from your question how that's usually being phrased by them), this is how I would initially respond.
I would frame this in such a way that makes it clear that you presume that covers everything that was asked adequately. Don't, yourself, offer to "go into more detail"/etc. Don't ask questions of what they're doing/wanting to do, unless you're actually interested in potentially fishing for work or at least giving them the impression that you do want to discuss this more. If you're speaking to someone who understands the field and was just generally curious, this will be enough to have explained "what you did" in a general way. If they want to start going on about a particular framework/etc, then you can happily jabber on about it too, or not.
I do definitely also recommend paying attention to the other answers which dive into taking the request initially as a compliment, and particularly answering back in a way that frames it as such and addresses the complimentary nature of it, not the request itself (beyond maybe what I outlined above of using a generic broad overview).
Finally, it doesn't hurt to mention that it was "a lot of work" and, inasmuch as it did "took a lot of time learning/took a lot of skill/was something you were glad you were already trained in ____/etc". Particularly any custom work as such. It's ok to be proud of what you did and how you got there, and it's ok to clearly signal the value involved. Answers describing pointing people to places to "get started" are appropriate here if the followup question is something like "wow, how did you get to that point?"
But if someone is still asking for more details, one eminently correct followup is to stand up for yourself, your related expertise, and your time as a professional, in a professional and polite manner:
Well, it would require more technical detail than I have time for to get into specifics, but I do consulting on this: here's my card and I'd be more than happy to schedule a consultation sometime if you're interested.
If you really want to make clear up front and right now that this isn't going to be free even then (although the language cue/signalling of using "consult" is intended to help here), you could follow that up with either something along the lines of
...get in touch, and we can discuss my rates.
Just follow the link on my card to my site, then it's under "Consulting" in the menu.
(have either a basic rate schedule or a mention of discussing rates on the related page)
There is nothing wrong with making clear that this is an aspect of your profession and as such it is work, and you are not planning on doing work for free just because someone else demands it of you, naively or otherwise.
Part of this is a matter of establishing clear boundaries with others. It's that you're still being friendly, but are not going to be taken advantage of for being friendly.
There is a difference between professional peer curiosity and related friendly discussion, and people who are effectively (even if unintentionally/unaware of doing so) devaluing your work and your skills in order to essentially ask you to do work for them, for free. Regardless of whether they realize that's what they are doing. Because, yes, standing there explaining in deep detail is work, even beyond the discussions in other answers of proprietary details or not.
Standing there and having a friendly peer discussion about some esoteric detail just because you found it interesting and the person you're talking to is knowledgeable on the topic and also finds it interesting is a very different story. I'm assuming that you can easily tell the difference between the two conversations.
Another part of this, and why I use the word "consulting" regarding this, is that the firmly structured context you are creating makes sure that this is regarded appropriately and also appropriately seen in regards to your own views. Do not suggest you would do this for "a few beers" or anything similar. Firmly contextualize this as work for you, not just a fun hobby you do for funsies, and make clear you expect others to treat with you on that context as a professional.
Finally, to be clear, the expectation is that once "real money" is involved, most people who have casually engaged you on this topic as if you are just going to magically be able to tell them how to do what you've done in the space of a conversation will at best express some interest in following up and then never do so. Firmly shifting this into a "I'll work for you, but this is real work if I do so" and "only for real money" drives away nearly anyone who had created a mental model whereby they trivialized this into something where simply expecting you to stand there and tell all they needed off the cuff was in any way going to both happen and going to result in them getting results like yours, just like that. My partner often remarks that if she had a dime for every time someone asks her about an idea for artwork and then runs away after a noncommittal response when given a professional quote, she'd be a millionaire.
What about just dodging the question?
One other avenue, obviously, is to simply say you're "not interested in going into more detail right now", but this type of passive demurral doesn't help in resetting and firming related expectations and boundaries. The "right now" can also easily be taken as a "oh, but you'd be happy to at a later date". Relating your discussion of those details to being a job you could be hired to do for them firmly contextualizes the nature of you going into more detail as being work, and all of the surrounding connotations.
One potential downside is that some people will see this as "money grubbing". Part of this is that it will simply occur because there are people who feel entitled to demand work of others and refuse to view it as such. Depending on how much time you want to spend and how acrimonious things feel, you can take the time to ask them what their profession is, then ask if they expect to get paid for doing their profession, and hopefully bring them around to understanding why you aren't going to consult (and I suggest sticking to using this term for anything about further discussion, as it is a hard cue that forces context) for free: if you want to further soften this--assuming it's true--you can always say that you'd be happy to do it at your "friend rate" but this is work for you and you hope they understand.
You can also lean on the part that this is your time off and that you like to have some at least reasonable work life boundaries, and this is part of how you maintain those because otherwise people keep asking you to do work for free on your time off. Saying it like this is fairly passive aggressive (particularly depending on the tone), but can be reworded to be nicer or not as the situation demands.
Some people will refuse to understand no matter what, and no matter how nice you are, and I suggest dropping or shifting the topic of conversation at that point. You can't fix everyone. And yes, to me, this is something that is worth describing as "broken" when someone else refuses to be civil about it.
I've seen this happen in a number of fields that involve intellectual and especially creative work. Some people have a mistaken impression that just because in broad theory "anyone can do it" (in terms of things like "anyone can arrange some images on a page!"), and what they see as the "work" involved is "so simple", that means that the time to describe or even outright do it is something which whoever they latch on to should simply grant them, and that anything else would somehow be uncivil. It's hard to pull apart the genesis of this behavior, particularly since it varies person to person.
While I think it is nice to try to find and correct the underlying faults in their schema leading to this type of impression, that's also a form of emotional labor for which I don't think anyone should feel forced to engage in, and can be a hopeless task that just leads to acrimony as they pull back and entrench. I also wouldn't suggest making a professional referral for related counseling at this point, unless you really intend to insult them (because that's almost certainly how it will be taken).
In Short (aka TL;DR)
I've personally found that simply setting clear, clean boundaries, explaining your own stance in a relatable way (e.g. "this is work for me, just like your job is work for you, and I wouldn't ask you to work for me for free, please respect the same of me"), and not worrying about "fixing" their related mental model, but simply calmly ending the conversation if they insist on arguing, is the approach that overall works the best, and if done correctly also makes you appear to have taken the "higher road" to anyone else who is present.
Aside from the IPS side of this, one plus is that it may actually generate some related work for you, assuming you're interested in that. While this answer might seem to presume that you would be, it's relatively easy to adjust this for not wanting to or having time to do said work, which involves either raising prices (and I generally recommend setting them firmly at a reasonably high scale either way) or simply saying you're too booked up for the forseeable future [but can refer the person to someone else who might be able to help (assuming that's true--and this can be a nice way to send work to friends/colleagues who freelance)].