From this guide: Building Apps for iOS with MIT App Inventor
i do not see posibility to buid ap for TestFlight...
is it even possible to build it that way?
Thnx
According to MIT, yes it is possible. The link is MIT's advice. Yes it is complicated. 
If your code works on an Android device, it might work on ios but that is not certain because all the controls that work on Android do not yet work in the ios environment.
I think Firebase is also not compatible with the app inventor ios version yet.
Yes, you upload your app to the App Store regardless. Deploying to TestFlight is done through the App Store Connect UI.