Little to do with Android, mainly about 3rd party apps.
I think it's at least partly covered on page 2 under Uncovering Parent Companies :
> We obtain the parent organizations of these services (after accounting for business mergers and acquisitions) and identify the dominant organizations in the mobile ATS ecosystem. We find that Alphabet-owned ATSes have presence in over 73% of apps in our dataset. This raises questions about Alphabet’s monopoly in the mobile ATS ecosystem.
So alongside their conclusion that information is shared freely between subsidiaries:
It would suggest that other large "players", along side Alphabet, could have much larger collections of user data than cureently understood.
That would be my guess though.