I want to back up my photos using the photos app, but I don't want some of them to show up when I open the app on my phone. Is this possible?
Install file manager
Locate folder where backed up photos are
Put a .nomedia file in that folder
(A .nomedia file is an empty file with no code, which hides media in that folder. To get it you can copy paste it from elsewhere, like if you have WhatsApp, you can browse to it's media in file manager, go to pictures or videos, then to sent, and it should be there, or just do a search in your file manager for .nomedia, or you can make the file yourself)
Go to settings, apps, media storage, and clear it's data.
Restart your phone, go to gallery.
You might need to wait a bit for photos and videos to show up, because you cleared media storage, but the photos in the folder with the .nomedia file won't show up.