Below is the source code for 2020+ SUBARU head units and other core functionality. Many modules written and others since other car manufacturers share the same code base.
You can also find Toyota version as well.
Here is the Subaru link:
I found it a year ago, made minor modifications and compiled successfully, however, to get it deployed is another large task which I didn’t follow through with, there are probably folks who have done so already, I am not certain.
Send me an email if you do. Cheers!