What user/group does WordPress belong to in terms of file permissions?

I think the real question you have here is — what user/group WordPress belongs to?

WordPress itself essentially doesn’t have a user/group. From perspective of filesystem it is just a bunch of PHP source files lying around.

What is really “has” a user here is your web server and/or PHP binary. Together they execute WordPress source files and any file operations are under their authority.

In a nutshell — owner and permissions of files that you want editable need to be configured appropriately for your specific server, which varies depending on the server. Obviously permissions need to be as narrow as possible (777 and such isn’t wise that is).