How do I share full-resolution photos between Apple devices?
Airdrop sends full sized versions.

iCloud link/album sharing sends full size versions.

iMessages can downsize if enabled in Message settings, but I don't know otherwise. 

Put them in a note and add collaborators or create a sharable link.

Put them in iCloud Drive and share from there.

Use Dropbox, Firefox Send, Google Drive, OneDrive.

Plug in the iPhone to a Mac, trust the device and import using Photos.app. And then export unmodified original or let it do so modifications.