I have not used these services, but it's very likely that they are since that's basically how all beta testing outside of TestFlight works.

You can use Ad Hoc deployments, which is what I used to do on TestFlight before it got acquired by Apple.

This requires registering UDIDs, right?

Yes, it does. For testing, in smaller business, this is fine however as you can register up to 100 devices. Enterprise certificates were much easier to use in big enterprise thought even if they weren't really "meant" for that purpose.