To favorite a trainer:
Navigate to the Workouts tab
Scroll down to the Trainers section
Tap on your favorite trainer
Tap on the heart icon
To favorite a workout:
Navigate to the Workouts tab
Tap the Search icon in the top right corner
Search for your favorite workout
Tap to open the workout details screen
Tap on the heart icon
To locate your favorite trainers & workouts:
Navigate to the For You tab
Scroll down to Favorites
Tap on Favorites to see all your favorites