Phones with Replaceable Accessory Modules

Interchangeable accessory modules are parts that attach to the back of the phone and talk to it electrically or magnetically: an extra lens, a light, a grip, a battery. They do not change the phone's own hardware; they add a capability to it temporarily.

On the camera side the most meaningful uses are extra optics and light: a grip module lets the phone be held like a camera, a light module gives soft light for portraits. The price is bulk and, usually, being tied to an accessory ecosystem specific to that phone.