The two companies have quite different world-views on the cloud. Apple tends to have user data encrypted in their cloud and to encourage more on-device process

The two companies have quite different world-views on the cloud. Apple tends to have user data encrypted in their cloud and to encourage more on-device processing. They are more likely to support data sharing between people via exchanged cryptographic keys than have access within their cloud. Unfortunately, user-initiated sharing delegated access out is not a flow OAuth really supports. Google's previous social network aspirations caused them to chain access to lots of data through user authorization to APIs. They use API access as one of the attractions to sites supporting "Sign in with Google". AFAIK even API which do not necessarily need any user delegated authorization (such as reading a publicly accessible google doc via API) still require a user authorization. Apple's APIs typically are not about delegated access to user data, but about access to user's application data (e.g. CloudKit app data access via REST api). Apple does have a OpenID Connect implementation for Sign In with Apple and have left the option open for it to also be used for OAuth delegated access. So far, there aren't any access token scopes or API publicly documented.