Since Unity version 2019.3 it’s possible use a Unity app as a library in a native mobile app!

Based on this example, I managed to add a Ionic app inside a native mobile Android app, so it can launch and communicate with the Unity app.

Using npm run ionic:build we get capacitor to copy the built files to /android/app/src/main/assets/public/ then we can copy those files in the android project or modify the directory structure to allow direct use.

Example repository