Deleting your repository or changing its visibility affects that repository's forks.

Sure, but this only matters for 'private' repos. See <https://docs.github.com/en/pull-requests/collaborating-with-...>.

Github does some cute things behind the scenes to save space, but for all 'public' or 'internal' repos, what Github calls a fork is (from a Github user's perspective) identical to what happens when you run 'git clone' on your machine.