I started iOS development with a number of so-called audio units. These are technically spoken app extensions for realtime audio processing, which are shareable across the eco systems. In the meantime most of these audio units are super-universal and available on nearly all Apple platforms, iOS, iPadOS, visionOS and macOS.
The audio unit specific website can be found here.
Note: On visionOS, audio units are (currently?) not supported for native apps. But such app extensions are available in iOS / iPadOS compatibility mode.