delete photos just from my mac

If you are using iCloud to synchronize your libraries, then deleting a picture from the Mac will delete it from all other devices, and deleting a picture from your phone will delete it from all other devices. iCloud is a synchronizing service, so what you do to one automatically happens to the others connected to it.

If you're not using iCloud synchronization, then the devices are independent from each other.

If space is a problem, you can save space on your Mac by choosing "Optimize Mac Storage" in Photos Settings (or Preferences.) This will store smaller resolution images on your Mac (they'll look the same) while the full resolution images will always be available through iCloud.