GitHub uses Git. Git itself is ill suited to Google's size. Years ago Google has tried something called git5 which aimed to bring git to the Google monorepo. It failed miserably because Git itself isn't scalable. Lest you think they didn't try hard enough, Facebook tried the same thing and came to the exact same conclusion that Git isn't scalable. Nowadays both companies use mercurial which is much better. They couldn't even switch to Git let alone a forge based on Git.