I will add though, some fields are far more open than others. Most notable would probably be software or computing in general. An observant person can glean insights from places like HN, Reddit, lobste.rs, GitHub/Lab threads, university course pages from all over the world, personal opinionated blogs, etc. etc. (add reading good open source code to this list too). I say observant because you need to sift through marketing (mostly HN and Reddit), influencer crap [0], (often attractive) polemics, etc. With computing, these places can come from genuine passion and respect for the craft, rather than grift like you'd find on LinkedIn [1]. From there a person can weigh up different ideas and experiences, looks for patterns, try to determine what is being implied and what context has gone unsaid, what is taken for granted, etc. This would replace otherwise unavailable mentors.
Like you, what you mention has been my personal experience. My experience with people from my demographic is that some, without formal guidance, struggle, and in getting started really need someone to hold their hand. In some cases this would be a problem of confidence and self-esteem more than ability. Others can soldier through looking to pick up bits of wisdom from wherever, eventually being able to make judgements of their own.
>and it makes me highly suspicious of anyone whose advice for acquiring technical skills is simply to practice constantly - "draw every day," "you have to code," "always be networking," etc.
It's a matter of perspective I think. This reminds me of that Ira Glass quote on taste and creativity. Perhaps taste is a sort of tacit knowledge, and this is picked up from mentors, or developed independently like I mentioned above. Then a person who does not know the experience of lacking taste -- either because they have access to mentorship (and took it for granted) or because they had the confidence and ability to independently "seek" tacit knowledge (and took the confidence and ability to actually do so for granted) -- lacks perspective and gives this advice. It "worked" for them, but really this is just the apparent, and they cannot express how they developed the taste or tacit knowledge (or that it is necessary!) for this advice to be useful.
[0] I specifically left YouTube off that list. Aside from conference videos posted to YouTube (hardly any views), university lecture recordings (hardly any views past first year with a few exceptions), and maybe a feeew other channels, YouTube content is in my experience garbage and very typical-influencer, mostly hyping up megacorps, the latest buzzword-tech, and grinding leetcode.
[1] Yes computing grift is very real, but I mean that it appears that sharing information online for "nobler" reasons or genuine professional reasons is more common in computing than in some other fields.