Seems like Duolingo apparently uses both? Or perhaps one of these is out of date.
>As Duolingo’s products grow and evolve to reach even more learners around the world, Lottie animations help us keep users engaged, delighted, and always learning.[0]
>We thought the answer might lie in an alternative to a game engine —something that can help us take a limited number of assets and turn those into a virtually unlimited number of combinations. This is how we learned about Rive![1]
>As Duolingo’s products grow and evolve to reach even more learners around the world, Lottie animations help us keep users engaged, delighted, and always learning.[0]
>We thought the answer might lie in an alternative to a game engine —something that can help us take a limited number of assets and turn those into a virtually unlimited number of combinations. This is how we learned about Rive![1]
[0] https://lottiefiles.com/case-studies/duolingo
[1] https://blog.duolingo.com/world-character-visemes/