It's funny how boomers don't understand Stackoverflow-driven development.
We get the most benefit by going against common sense and popular opinio...
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I agree with your statement on 'copy what's known to to be the best'. For me that's usually syntax, an idiom, a 3 liner, a neat algorithm. Small things. However I can see the ivory tower effect popping up after one learns these ( with or without SO), applies them with success and realizes that these were the small ones and there're way bigger problems to tackle which don't have a public answer. If you combine this effect and that many developers walked this road without Stackoverflow you may see why some more experienced devs say that the copy paste approach won't take you too far. So they have a point there, but I believe that you (we) are also right about the 'copy what's best' or 'follow the guideline' principle.
Wow, the most amazing thing here is that you commented on this post. I never promoted it, even on twitter, but still.
Returning to the question, I think that the development process is a very interesting thing with multiple layers of complexity. It's like a network service security - if you think that the service is either secure or not, you're doing it wrong. You have to establish some threat model or risk model in this case, and then continue carefully with a full understanding of what and why you do it. Every layer has to be solved separately with different priorities. Failing to do that leads to learning every algorithm of the world without any actual leverage to use it in practice in real life, establishing R&D departments that do R&D on nothing interesting, and different kinds of corporate cringe
I agree SO is the most rational option but if you truly want to learn documentation should be your first choice!
Maybe it worth contributing something to truly learn the thing. Or maybe, creating a video course or blog. You're learning much quicker when you actually try to explain or change the internals of a technology