If I login to a mobile/web app with a Google account, what stops the developer also using that account?

I have an admittedly-vague understanding of how OAUTH works, but I recently went through the rigmarole of installing rclone and having it sync some files from my Google Drive to my Linux laptop.
As part of rclone’s documentation, it is rec… Continue reading If I login to a mobile/web app with a Google account, what stops the developer also using that account?