You cannot view iCloud Backups in a web browser. The data that you can view at iCloud.com is synced data (e.g., iCloud Photos, iCloud Drive, iCloud Contacts, etc, but not iCloud Backup).
If you have 2 devices using the same Apple ID, all synced data is the same on both devices and at iCloud.com. However, the iCloud Backups for each device are stored separately.
If you want to delete a backup for one of the devices, you can go to Settings > [username] > iCloud > Manage Account Storage > Backups > [device name] > Delete Backup.
Also see: Manage your iCloud storage - Apple Support
Removing a trusted number does not affect any iCloud Storage.
If you want to sell or give away a device, you can use the following procedure to remove all data from that device without removing any data from other devices or from iCloud: What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support