do I do the course challenge at the beginning or end of finishing a course?

You can do them at anytime! Some people might know certain parts already and can get credit for them, just like it will lower your mastery percentage when you answer incorrectly - I found that part out the hard way haha

I stick to the with the unit quizzes and tests now. Those stick within concepts in the unit, with the quizzes including everything up to each quiz.

Hi chen.adora.z!

The Course Challenge is a great way to get credit for skills that you are already familiar with or have already mastered when starting a new course, so I recommend taking it at the start if you are reviewing a course that is already somewhat familiar to you.

If the course is totally new to you, or if you're planning on going through the entire course anyways, then taking the Course Challenge at the start probably isn't necessary.

Taking the Course Challenge at the end, however, is highly encouraged, since this is a good way to review all of the topics covered throughout the entire course to make sure that you have truly mastered everything.

I hope this helps!