The Photos app opens whenever I plug in my phone.
How can this be prevented?
I have seen many solutions for the old iPhoto app that involve changing the "Connecting camera opens" option, but the new app has no such option.
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.