Google’s main revenue is ads.

Same goes for their other services.

I’d also assume with many ad-supported apps they’re also leading source of ads (also on iOS)

Another point to consider, Is they DO take fee from each device developer that includes Google App Services. So basically ALL devices with official Play Store sold by the manufacturer already pays a fee to include their store (but also that’s the only way to have official Gmail and other services users would expect when buying an android device)