1. If you have Google Drive for PC/Mac installed, it will update it to Backup and Sync.

2. The difference in the two endpoints are the defaults selected during the setup wizard (which you can change from either). The Photos versions is geared more towards Photos users and the Drive users more towards Drive.

3. They are saved in Drive, and are synced to Photos. However if you delete in either Drive or Photos cloud, it will affect the other. Whether it is deleted from your hard drive depends on what you have selected in the deletion settings portion.