If anyone has an edge on them in the cloud they can possibly come out with competitive offerings to theirs similar to how companies using TSMC has done to Intel.
Google's infrastructure is a cloud, the only thing a competitor needs is the software / services to replace Google. And a lot of them are already there, being recreated.
That's why they want in, because Google & co. could easily become the next television.. Its still there, but most people don't care about it any more because of a shift in technology and the way people use the internet.
I don't believe that's a true characterization; not any more correct than considering Search to be a GCP customer. Not true technically nor in terms of revenue booked. A more accurate description would be GCP and YouTube both are built on top of common Google infrastructure (Borg, storage, networking, monorepo/dev infra, etc.)
No idea how true is the latter either, Amazon being an AWS customer.