how do I get my photos on icloud to my partner's photos?

There is no option to automatically sync all of your photos to another Apple ID. The closest option to that is a shared album. See the following from Family Sharing - Apple

When you set up your family, a shared album is created automatically in the Photos app on all family members’ devices. Everyone can add photos, videos, and comments to the album whenever they like and get notified when something new is added.