Not in terms of revenue that YouTube or Play generate, but in terms of YouTube / Play being a customers of GCP, themselves. In the sense that Amazon are one of the biggest AWS customers.

> in terms of YouTube / Play being a customers of GCP, themselves

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.

It isn’t. I listened to a podcast episode of “screaming in the cloud” where a GCP engineer said that only a few internal Google tools are hosted on GCP.