How do I know how much I have stored in my iCloud Account - and how do I see it?

See instructions in: Manage your iCloud storage - Apple Support

Also note that on an iPhone, iCloud is designed to be viewed by using the apps on the device (e.g., the Files app for iCloud Drive, the Photos app for iCloud Photos, the Notes app for notes, etc.).

You can also use an internet browser and log into icloud.com. What you can access via a browser varies by device. See: https://support.apple.com/en-us/HT210402

While you can view synced data as described above, it is not possible to view the contents of an iCloud Backup. You can only use the backup to do a full restore of a device or to setup a new device.