I’m trying to find out which is the latest version for the ios companion app and 2023.4 is what I see in the app store and the GH source code.
I assume these are the right places to look for: is this correct?
Guess so - where else? If you are not jailbroken all the truth you can find/get is anyways limited to the 
-store - even if github would host a 2024.1 source code/binary already 
Hi, I have very limited experience with products of the fruit company.
Somehow I found v2023.9 in the app but of course the info on those pages did have the right version info and it became clear just now that this is the HA version number. 
You can take a look into the testflight version, there is a newer version. But keep in mind, that it can have some bugs.
Current Version is 2023.7 in testflight.