You already know what you need to know. Though you will have a learning curve as you learn to use what you know. Practice and research is the answer to that.
1) Specifying the .aar in buildozer.spec is what causes the .aar to be included in the app.
2) The Pyjnius autoclass statement imports the Java class, an overview here:
Though you probably don't know the following, which is a possible pitfall:
Before you start check that the Java version used to build the aar is not newer than the Java version you installed for Buildozer.
If the arr is built with a newer Java you will get an error message similar to this: