How do I transfer photos from old to new iPhone 16 Pro Max?

If you traded in the iPhone for the new one, did you make a backup, before you traded it in and erased it?

Then you need to restore the new iPhone from the backup of the old one: Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

However, an iCloud backup of your iPhone does not include the items, that are stored in iCloud. If you have been using iCloud Photos, you have to enable all iCloud Photos services on the new iPhone that you have been using on the old iPhone, so the photos and other iCloud data can be synced back from iCloud to your new iPhone. And it is essential, that you do not erase any photos or other iCloud data from the old iPhone, while it is still syncing with iCloud, or you will erase the data from iCloud and from your new iPhone as well.

Do you still have the old phone?

Do you use iCloud? If so, what do you see at iCloud.com?