I guess my implied question should be stated more explicitly: is GitHub's front end code entirely separate fro the Rails code? That's not how Rails apps usually work, I thought.

Admittedly, it's been a long time since I looked at Rails code, and I don't have the slightest idea how GitHub is actually architected. But I don't remember the "front end code" in Rails being a separate thing from the server code, typically: the whole point was server-side rendering. Is GitHub using one of the JS/TS frameworks for the front end? I do remember that being a developing trend, a few years back.