In the course overview, you can use the filter function at the top right to select questions from different quizzes and course units. For instance, you can pick questions that you have answered wrong or questions you have marked as favorites.
The filter can also be used to select questions across several quizzes and course units (a course unit is a hill, and a quiz is a rectangle that isn't a "speech bubble"). Just click on the filter symbol, then click on the rectangles you wish to pick.