The app is built using Swift programming language and Xcode. The Quizzler app is a trivia game that allows users to test their knowledge by answering a series of multiple-choice questions. The questions and answers are stored in an array of Question structs and are accessed by the QuizBrain struct which handle the logic of the quiz. The app displays the question and possible answers to the user, and when the user selects an answer, the app checks if the answer is correct and displays the result by changing the color of the button. The app also keeps track of the user's score and progress.