GarageBand is free, and is not too bad for recording and mixing with other instruments.
The audio & midi on iOS is actually implemented pretty well, so most apps that support the standard (the name escapes me now) can communicate with each other.
Definitely check out the Moog apps (Model D, Model 15) - they used to cost like $30 but ever since lockdown started they were free (hopefully they still are).
There is a very long thread on here called iOS Music Apps or something.