Actually, what is the reason that Firefox seems to be preferred over Chromium? Is it the license? The control Alphabet has over it?
One has to agree that there is a lot more money poured into chromium, the code is more modern and easier embeddable, it is more feature-complete.
Though, it’s good to have two independent browser engines and a non-profit (+for-profit subsidiary) dedicated to a free, open, user-focussed browser.
https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/about-forks
It’s basically like a copy of the original repository. But you can pull in and merge changes from the original, make a pull request for the original to pull your changes. Fork+pull request enables you to contribute to someone else’s repository. Things like Chromium are in part forks of Safari, just that they diverged over time.