How to get photos and videos after iPhone reset

When I buy a new phone (very rarely,) I rely on iCloud backup, iCloud Photos, and the backups I send to my Mac, just as muguy says.

If you don't use iCloud Photos, but you have Settings>Apple ID>iCloud Backup turned on, then you may be able to recover data. You should look here:

Restore your iPhone, iPad, or iPod touch from a backup - Apple Support 

“Restore” is what you do when you buy a new phone, so this safety, if you have it set, requires you to erase everything on your phone and replace it with the backup.

Several backup options are discussed here:

iPhone backups--iPhone User Guide

Unfortunately, if you don't keep your own backups to an external drive, then there really isn't any place else to look.

If you reset your device any content is erased. You can recover from your backups or sync again to iCloud after logging into your account.