Confused about application area of Android bundled tool "aidl-cpp"

Question

I am a novice in Android IPC (actually I used Unix domain sockets for communication long ago circa, 2014) and now it would seem that I need to learn AIDL/HIDL to understand the "recommended" IPC mechanisms.

Now, coming to my doubt, I have a simple requirement (more like a pet project, an academic one, so to say) to teach myself how IPC works in Android 10 and up.

I want to build a native service (c++) which will be started from say, adb shell. [SELinux and other security implications can be ruled out]

Next, I want to Make a simple Java App which will have a Native library (via JNI).

This native lib, when signaled from App, will spawn a new thread, which will then attempt to communicate with Native service and fetch some data to be displayed on the App.

In a nutshell, the following is what I want to make to teach myself

• Android App dev
• Android JNI, Native interface
• POSIX threading in Android native code
• Android IPC [binder]
• Developing an Android native service as well.

A rough sketch would be something like:

App <--> JNI <--> Native lib <--spawns-thread--> Native Client <--IPC--> Native service

The IPC would be via binder and for which I want to learn and understand the basics of AIDL.

I have gone through the link: https://android.googlesource.com/platform/system/tools/aidl/+/refs/heads/android11-dev/docs/aidl-cpp.md

However I am confused in multiple areas such as:

1. When to use "aidl-cpp" to generate the Bn/Bp interfaces? Can I simply allow soong build system to generate the required files?

2. If #1 is not necessary then how am I supposed to write the Native service code without Bn interface generated in first hand?

3. How can I tell "aidl-cpp" to compile multiple .aidl files?

I have also gone through another link: https://github.com/airgreen/nativeservice-app-aidl It is very well written but does not exactly answer my questions

Any help on the above topics would be very helpful.

Thanks.