I want to back up my pictures to see the other pictures on my device
I want all my pictures on iPhone 12pro to be on this device
iPhone XS Max, iOS 13
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You can turn on iCloud Photos on both devices. You may need to buy additional iCloud storage first,
Or you can store all your photos on a computer and sync back to both phones manually
Manage your photo and video storage - Apple Support
There are a number of links branching from that link