Do Gallery or Google Photos create extra copies of media from other apps? I have a lot of media files on my phone: some of them are sent and received by WhatsApp, others are made by my camera. I configured Gallery application to see  WhatsApp media directories and I use Google Photos to backup all my media files. Now here is the question. How many copies of the actual file are created on my device? Does Gallery create a copy or just list the files? Does Google Photos create a local copy or just list the files?
You will have only 1 copy of each media files on your device.
The media files will be stored in the default location i.e:- File Explorer.

The media files will also be stored on the google server since you have completed the backup. But it will be virtual for your device. 

Note:- Gallery application is only used to display the various files stored in the memory.