Your favorite songs can be added to the "Favorites" list (accessible under SONGS --> Favorites). Please see below how to do that.
1. Choose a song:
2. Play the full version of the song (earning stars is not necessary):
3. Click/ tap "Add to favorites" icon on the OVERVIEW results screen:
4. Voilà! The song is added! Clicking/ tapping the same icon will remove the song from favorites.
5. The added song can be found in the "FAVORITES" list under SONGS --> Favorites: