1. Yes, that is correct, if repositories of proprietary repositories is asked (on distros that only install open-source software by default, like Fedora), you should enable that for Nvidia and Steam. This won’t have any downside really, it just lets you install a few proprietary/closed-source software. It won’t even significantly take up more space either, as what it’s essentially doing is telling the app installer manager to check these URLs in addition to the default ones when searching for things to install. If you don’t select this option then you won’t be able to install Nvidia drivers or Steam without manually adding the repos in the menu hidden somewhere. If you are going with a gaming specific distro they might also have an Nvidia-specific version for you to download.

2. LearnLinuxTV, the YouTube channel, is super useful, as well as Veronica Explains (who has both Peertube and YouTube channels)

Bread on Penguins is another channel I follow and they recommend a bunch of neat little command line tools and tips and tricks regarding the shell (like RMPC, which I really like as a music player!)

Most importantly, always read the documentation! Of your distro and of the software you are using. It can really help a lot!

Another thing, even if you don’t use Arch, ArchWiki has excellent documentation on a lot of things that apply to all Linux users.