Photo app

Acccording to:

Delete built-in Apple apps on your iOS 12, iOS 13, or iPadOS device or Apple Watch - Apple Support

you should not have been able to delete the Photo app.

The first (and simplest) thing to try if something isn’t working right is to restart your iPhone. If necessary (device not responding), try a forced restart. Both methods can be found here:

Restart your iPhone - Apple Support

If you have a MacBook, you can simply use an data transferring cable or an apple charging cable and connect your iPhone to the MacBook and backup your iPhone or try restoring it. If it doesn't work then try restoring your iPhone and that will mean that your iPhone will be in factory reset.

If you do not have a MacBook, simply make sure your iCloud backup is tuned on at all times and it has space to backup your data, after that just erase the phone's data and it will be in factory reset.

All the best!