to answer your questions

yes, your whatsapp backup uses your google drive storage, it used to be free unlimited many years ago, no longer

if you're using an iphone, it uses iCloud

no, you cannot use Yahoo Mail

it is strictly iCloud on Apple devices and Google Drive on Android Devices

it is not related to the email you used when registering, but rather what device you use when you try to backup

Yahoo can give you 1TB because they don't have a cloud storage option and they know you probably wont end up with 1TB of emails