There is some sort of vendor lock-in VSCode. It at least used to be extremely difficult to make GitHub Copilot to work with codium. There is something closed source in VSCode that makes the difference.

It was so difficult to maintain, that I ended up switching to VSCode. So the ”lock-in” worked.

vscodium and VSCode forks are legally prevented from using the normal VSCode extension site. They have their own: https://open-vsx.org/

As far as I know Chrome forks are not blocked from using extensions from the Chrome Web Store.