Intersects quite heavily if you're defining a schema for your API

No, they're both possible choices for your basic client-server communication layer that you build everything else on. (I mean, technically gRPC rather than protobuf, but protobuf is the biggest part of gRPC).

What are differences ?

Yes. The use cases are very different, as far as these things go. To say otherwise is borderline misinformation.

You can build services internally with gRPC and serve a public graphQL API that aggregates them.

Not at all.