plolock
Facts:
Apps can communicate with each other.
IPC - inter process communication is it called.
There is no ipc communication permission yet. (Private space / user profiles is for blocking apps from unsing ipc with each other)
It is up to the app developer themself to allow its app to communicate with other apps/ they implement what this means.
I am not 100% sure about this part, but i think it is like this (that is how i understand the play service dependencys of apps):
And a lot of apps use play services, which essentially means the use ipc to communicate with it.
For example they could use it for the notifications.