Reading (many) driver manuals, inputting questions from YouTube videos of people who videotaped their printed test, receiving questions from users, receiving questions from other third parties. While some of the questions are common, I can assure you majority of questions are unique (eg - different state rules, differnt questions for car vs. truck vs. bus, etc), so the work required has been significant to do this.
The copyright is our own. No copyright issues as we are not copying and instead coming up with a question from a paragraph of text from a manual explaining a particular concept. This is how we derived the vast majority of the questions.