Q: how to transfer photos from desktop Photos to iPhone

I can easily transfer photos from my iPhone to the Photos app on my MacBook Pro. Now I want to send some back to the camera roll on my iPhone from Photos. My iPhone doesn't appear as a destination to save to. (I am not using iCloud due to limited data) Is there any way to do this directly from within Photos or will I need another step. I did it once with wifi transfer, but it means means exporting and then switching to another app, etc. It seems improbable to me that Apple would make the iPhone - Photos transfer a one way trip.