You can now sync your favorite photos between Apple Photos and Google Photos, according to a new support document from Google Photos. You must be using an iOS device for this feature to work.
How it works ● Your favorite (heart) photos in Apple Photos become favorites in Google Photos. ● Your favorite (star) photos in Google Photos become favorites in Apple Photos. ● If you unfavorite photos in Google Photos, it unfavorites photos in Apple Photos, and vice versa.
You can enable the feature from the Google Photos app on your iOS device:
Turn sync on or off ● On your iPhone or iPad, open Google Photos Photos. ● At the top right, tap your account profile photo. ● Tap Photos Settings Settings. ● Tap Apple Photos. ● Tap Sync favorites On. ● Tap Turn on sync or Turn off sync.