If you have a Mac and xcode on it you build the IPA and deploy it to your iphone thru xcode. A developer needs to be able to test their own application this xcode just uses your regular devices and let’s you deploy onto them.
The beauty of open-source is that you can build an APK/IPA yourself and be sure that your app - your version is truly not spying on you.
I think most people just take Signal’s devs words for it that it’s the same.
My Grandpa used to say “Every farmer praises his pond.”