Since Motif is a native extension to Photos on macOS, it runs directly inside Photos.
Once you download Motif from the Mac App Store, Motif will be automatically plugged into the Photos app, allowing you to create memories using your Albums, Smart Albums, Memories, and other image collections. You do not need to export and import your existing photos into another photo application outside of the Photos app on macOS.
To access and launch Motif, please follow these steps:
From here, Motif will open inside the Photos app and allow you to choose and launch the type of photo project you want to work on, such as a photo book, a calendar, or a set of cards.