How to transfer photos from one iPhone to another?

Wirelessly via AirDrop or via sneakernet with a USB-C drive and Files.app. The latter method might be more robust and fast especially if you use a proper SSD drive instead a cheap thumb drive.

USBc will be problem on an iPhone 11, and I don't remember iOS 15, much. I had to look up "sneakernet." It apparently means putting stuff on a drive and then "walking it," apparently wearing sneakers, over to the other device.

AirDrop is really great, but AirDrop is made for doing a few pictures at a time. So it's probably best to divide up a big bunch into smaller groups. This

How to use AirDrop on your iPhone or iPad - Apple Support

doesn't discuss a maximum number, but we've seen people have trouble with doing lots at once.

If the sender is using iCloud Photos, then a shared link can be set up:

Share photos and videos with an iCloud Link on iCloud.com - Apple Support

I tried just connecting my iPad with my iPhone-- I got "Trust this Computer," but they don't appear in each other's files app. Darn!

Probably DropBox or the equivalent is one of the better options...

are both phones on the same Apple account? Or are you trying to transfer a large number of photos and videos from one Apple account to another Apple account?