It's not the same code base though. They're all different branches, and also differ in code (although not by much, but it still requires manual maintenance of each branch). I haven't seen the actual build process but it's likely to be completely separate CI/CD pipelines, so I wouldn't claim it to be the "same" build process either. Also, Focus uses a completely different UI with a different/cut-down set of features.
Naturally I'm not saying that maintaining these branches amounts to the same level of effort as maintaining the iOS WebKit and Gecko branches, but it's not some non-trivial effort either.