There are two main types of Smart Trainer workouts.
ERG mode keeps you accountable during interval workouts, making sure you hit your power targets by increasing resistance if you slow down too soon, or easing up if you pedal faster. ERG mode apps often come with libraries of personalized interval workouts, and show your training data onscreen. Gearing doesn't change the difficulty of your workout here, so leave your bike in your fastest gear for the best road feel.
SIM mode simulates an outdoor ride: tougher on the climbs than the downhills. SIM mode apps often come with realistic graphics and allow you to race with other app users and participate in challenges. Use your gears like you would outdoors.