Instead of picking specific units to save, the Duolingo app now automatically downloads a limited amount of content based on your current progress in the learning path.

Ensure you are on the specific language course you want to practice. You cannot switch languages once you are offline.

Lessons are typically downloaded in advance when your device is idle.

Today, downloading Duolingo lessons is primarily an that relies on background caching rather than a manual "Download" button. How Duolingo's Current Offline Mode Works