Duolingo's platform is powered by a combination of machine learning models and, in its premium Max tier, GPT-4-based AI features including "Explain My Answer" and "Roleplay" for conversational practice. The core learning engine uses spaced repetition algorithms and Bayesian knowledge modeling to predict and reinforce weak vocabulary and grammar points. The mobile apps (iOS and Android) are the primary delivery surface, supported by a responsive web app. At the infrastructure level, Duolingo processes millions of daily active learners and leverages large-scale A/B testing to continuously refine its pedagogical models.