That was always the best thing about old days demoscene, specially at the parties, having one group proving that something deemed impossible on platform X was actually possible, and then having other discovering how they might have done it, and topping it with something even better.
The game dev culture evolved from the demoscene.
Nowadays due to the FOSS influence, there are some people sharing, but they are mostly the exception to the rule.
I'm not sure that's the case. The demoscene really started with the Amiga in the mid 80s while game development was already its own thing. Maybe they influenced each-other but I wouldn't say one evolved from the other.
https://en.wikipedia.org/wiki/DICE_(company)
Back in the Amiga days there did seem to be a bit more migration of ex-demo coders becoming game developers. Probably Team17 were the best known.
The papers tend to cover everything from general implementation, to details such as cache optimizations they discovered later on in development. If you reach out to the devs over twitter, they're often more than happy to answer any questions that you may have.
Not open by foss plug-and-play standards, certainly. But technologically, they're absolutely open.
Frostbite technicaly solutions: https://www.ea.com/frostbite/news Rendering research papers: https://www.ea.com/seed/publications
You also don't get to learn about anything that is game console related unless you get access to the Nintendo, Sony, Microsoft, Google internal dev forums.
Finally, yes there is stuff like GPU and Game Gems book series, but usually when one mentions that, many seem allergic to buy such books at 50+ € a piece.