Will iCloud Photos combine or override/replace what is already there?

Hi, so I have two devices, a phone and a mac that has the same iCloud but don't share the same iCloud photo library yet.

I want to turn on iCloud photos to save storage but Im worried that if did that then either the mac or phone will replace those photos instead of merging them.

If I check the top box iCloud Photos, will it combine or override/replace what is already there on both devices?

Don't want to lose any special photos