What actually gets backed up, how many times? Say I have a photo A taken by the phone camera. It is stored somewhere, but can be seen in the stock Gallery app as well as the Google Photos app. Q1:Are both apps using the same physical copy or are there two copies?

Then, say, my WhatsApp has a photo B I received in a conversation. It is I presumed stored in /WhatsApp/Media/whatsApp Images. It can also be seen in the Gallery and Photos apps.

My WhatsApp has backup set to Back up to Google Drive Daily using Google Account X. Q2:Is this a special arrangement between Google and WhatsApp, because I cannot see anything regarding this in https://drive.google.com.

In the Photos app, Settings, Back up &amp; Sync, the BACKUP ACCOUNT is Y, my other Google Account I have configured on the same phone. In the Back Up device folders details, WhatsApp Images is enabled. Q3:Does this mean that my WhatsApp photo B is backed up to my Google Drive in account Y as well as the special WhatsApp backup in Google Drive in account X?
Q1:Are both apps using the same physical copy or are there two copies?


They are both the same copy. Both apps scan the location the photo is stored, and the location contains only 1 copy of the file. If you have 5 copies of the same file, in the same folder, both apps will show you 5 photos, not 10.


  Q2:Is this a special arrangement between Google and WhatsApp, because I cannot see anything regarding this in https://drive.google.com.


I presume yes. On Android, WhatsApp uses Drive for backups, while on iOS, it uses iCloud.


  Q3:Does this mean that my WhatsApp photo B is backed up to my Google Drive in account Y as well as the special WhatsApp backup in Google Drive in account X?


Yes. Although the accounts are differents, both apps have been configured to access the same folder (in this case, WhatsApp Images). Thus, when backing up files, they both backup the same files in both accounts.