Despite instructions and links to the community forum on the GitHub repo [1] it is almost certainly a complete waste of your time to bother getting to know how to build, modify, and submit PRs to Signal for the iOS app.
Good quality PRs languish for years, half-ignored at best, despite comments from users and even ocassionally Signal employees that they are looked at.
The iOS app is in dire need of some improvements which many users have been contributing only to have their time completely wasted. I myself contributed a small change that would make it practical to delete all media from a chat.
I'm witholding donations until this situation improves.
[1] https://github.com/signalapp/Signal-iOS