As long as you're connected to a network, the selected collections and any operations you perform on the images on any platform are automatically synced among any logged in device. You can set the app to sync only over Wi-Fi as well. If you want to be able to work on photos while disconnected, you must enable offline editing on that device for the entire collection; you can't pick photos and just cache those. Otherwise, it dynamically downloads each image to your device as you open it and caches it temporarily.