Either I can’t find it or there’s no “fork” command on the “mobile view” of GitHub and they seem to have removed the option to switch to the desktop view (wtf, folks, it’s 2020, this is not how you design a web site).
Anyway, so one way to fork a repo from a phone is to choose the “Create new file” command. That will fork it for you (you don’t have to actually complete the process to create a new file).