What DAW are they using? I use my iPad as a remote for Presonus Studio One sometimes, but I couldn't consider using it as the recording system itself, since the AU and VST accessibility isn't there.

I would also use my iPad as a synthesiser (eg. Magellan) but then record the audio to a "real" computer, ie the Mac, via a recording interface.

you might enjoy this article: https://sixcolors.com/post/2019/02/a-week-of-podcasting-with...

GarageBand is enough for a lot of people. There’s also Cubasis and Gadget 2 (which is more synth focused).