Bishwas-py well, not exactly. It's just a mechanism offered by GitHub so you can work on the code.
Code ownership is different topic https://github.com/flarum/core/blob/master/LICENSE
Since Flarum is MIT licensed, you technically can "own" a fork as long as the license terms are followed (which is, keep the original copyright notice).
But there are also repositories on GitHub that have very restrictive licenses or no license, in which case Forking is restricted to what is allowed by the GitHub terms of use https://docs.github.com/en/github/site-policy/github-terms-of-service#5-license-grant-to-other-users For example my Audit Log Free extension has a proprietary license, but since I have put the code on GitHub users are allowed to fork it, yet not re-distribute it or appropriate it in any way. And I think you're not allowed to edit the fork either since the GitHub terms don't list that.