GitHub is the most mainstream, Gitlab has the most features and is selfhostable, Gitea has fewer features, but is more lightweight for selfhosting. Both Gitlab and Gitea are also working on federation.

I don’t know about the others.

They can, but they get untangled once OP has them again.

Wrong, there’s not a single Arch user who isn’t transfem, so none will look like that (/j ofc)