I’ve done some limited research, and it should be possible to build from anything if you don’t use any apple specific cpu features or frameworks etc. that being said, that will be a pretty bad experience so I assume these services require a Mac for that reason. I could be wrong about react native though, only found one questionable source that said you could.

I don’t know, never used it. Just know that it exists and can do that.

Not completely true. The best by far is xcode, but swift is open source so if you develop your own toolkit instead of SwiftUI and replace all other proprietary *Kit then any old computer can do it. You can also use (usually cross platform) frameworks which have already done all of this like react native which makes it really easy to do in other OSes, however you will still only get the best experience using native libraries which can only be built from Xcode on a Mac. Maybe darling will open the doors to running that on Linux one day though!