Apple - Prevent Photos app on OSX from launching when plugging in device

With the iPhone plugged in, open the Image Capture app on your Mac. In the Image Capture window, left lower corner, choose which app should automatically open when you plug in your phone, chose "no application"

If you like the command line or need to have all devices not open rather than manage each single device as it connects and then after Photos has already opened, try:

defaults -currentHost write com.apple.ImageCapture disableHotPlug -bool YES

This sets a flag to tell the system to not activate ImageCapture or Photos or iPhoto when a hot plug of an iOS device is detected.

Plug in your phone and let Photos load. Then note check box in the Photos window and uncheck it: