how do I remove photos from my iPhone but keep them on my mac

Sync: Keep your data visible and available on all devices that you access to. Delete on one device will delete on all devices. Sync your photos manually with iTunes. Each time that you sync your iOS device with iTunes, the photos and videos on your iOS device update to match the albums on your computer.

Import: Store on Mac / PC / External HDD for long-term (Risk of HDD corruption / malfunctioning) Or Import from Mac/PC to iPhone. Photos and videos that you sync from your computer to your iOS device using iTunes can't be imported back to your computer. 

Watch the GIF below for Sync and Import Process between Mac and iOS devices

Import to Photos app on a Mac --> Import photos from a camera or phone using Photos on Mac ...