That is true. But I reckon those big players just said "let's go with the flow and move some stuff to Github, good from the marketing point of view, etc". My point being that is just a trend. If Github is not careful, that trend can change anytime. Debian and GNOME are well known, big open source projects. This helps them a lot and pushes that trend a wee tiny bit in Gitlab's side.

People/companies go to GitHub to be social. It's the Facebook of computer code. That's why all the big players are there, so they can share/market themselves. It's also where a lot of programmers put their code so they can show it for interviews. That's why there are so many public repositories there.

Totally agree with this. This is the reason that I self-host with Gitea but mirror everything to Github.I essentially use Github as a forwarder to my Gitea instance.

However, I have seen a drastic decrease in interaction (PRs, issues, etc) with my self-hosted repos even though you can auth with your Github credentials.

Yes but how many of those are just mirrors and sources for non-contributors to find the code. See Linux Kernel as an example, the Kernel is mirrored on GitHub but none of the actual development takes place there.

I don't think it's about mirror of source code being there, I think it's more about a community of people and developers being able to report issues there. Eg. steam client is not opensource, but it has github repo where we report distro/Xorg/Wayland/libs related issues, and they often respond.

Most of these companies projects on Github are projects of their employees, not official projects :)

Some of these use GitHub for their open source work but mostly GitLab for their closed source work.