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
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