How do I back up my photos on my iPhone

Not quite sure where this question is headed...

Are you migrating to a new phone? Or starting out new with this phone, and not transferring existing contents from an older phone?

If iCloud Photos is enabled, your photos are copied to and stored in iCloud, and copies are cached locally on your iPhone. (Note that this means if you delete a photo locally, it will be deleted everywhere; in iCloud, and any other iPhone or Mac or iPad using the same Apple ID and sharing iCloud Photos access.)

Copies of what you have stored in iCloud can be archived locally, if you want a local copy:

Archive or make copies of the information you store in iCloud - Apple Support

If you have photos on your iPhone and aren't using iCloud Photos, you can enable that feature, or can import the photos into a photo library on Mac or Windows computer, and backups of that Mac or Windows computer will (further) back up your photos. If your library is on Mac or Windows, you can then sync those photos stored on Mac or Windows back to your iPhone, if you want.

Set up and use iCloud Photos - Apple Support

Transfer photos and videos from your iPhone, iPad, or iPod touch to your Mac or PC - Apple Support