oAuth is the authentication method used to get your token for access. You don't need to create a client ID/secret to authenticate.
Regardless if you use your own client ID/secret, you have to oAuth authenticate to get access to the drive API. The only difference is if you use your own client ID/secret, you use that when you make API hits. If. you use the default rclone client ID/secret (which is hard coded in by default if you pick nothing), you use the shared API.
If you make a service account, that uses it owns internal client ID/secret for the API.
If you use your own client ID/secret or service account, you can see those API hits in your admin console.