They may be paying to be the first option in "choose your email provider" lists. Although Apple's Mail.app (both iOS and macOS) offers their own and Microsoft's

They may be paying to be the first option in "choose your email provider" lists. Although Apple's Mail.app (both iOS and macOS) offers their own and Microsoft's services before Google's. For Google, the easiest way to gain new GMail users is to not deliver mail coming to existing users from 3rd party services or to throw it into trash/spam folders for no reason.