Photos from my Mac have been copied onto my phone.

If you are using iCloud Photo Library, then you have implemented a set up that houses your photos in iCloud and syncs those photos to all Macs and devices signed into that same iCloud account. These support articles should provide you with more information about how iCloud Photo Library works:

iCloud Photo Library FAQ - Apple Support

iCloud Photo Library Help - Apple Support

Cheers,

GB

That is how ICPL works - it keeps all devices in sync - for devices with limited storage use optimized photon on that tdevice

LN