I am trying to learn more about probability mass functions, density functions, expected value, and variance. Are there any online materials or quizzes (with answers and explanations) that I can use to test my understanding?
In my experience teaching intro probability courses, many online instructional materials (even trendy and popular ones) are quirky as to content or hastily written without proofreading. (I have spent many hours over the years dealing with student confusion due to reading poorly written online material.)
I don't want to deprecate online probability instructional materials en masse, but I don't have any suggestions that I can recommend with confidence. If someone else has a recommendation based on personal experience, perhaps it will appear in another Answer.
If you are looking for something that's free online, then I can suggest Grinstead and Snell's 'Intro. to Probability'. It is available in book form, but you can find a free PDF online. Look at Chapter 6.
There are many exercises at the end of the chapter, but I don't know if an answer book is available. However, there are many worked examples. You can choose to view some of them as problems, if you don't peek at the solutions.
Grinstead & Snell is competently written and well-reviewed. It has some examples that are not standard for elementary texts (which was sort of the point of their writing project). Also, the book uses simulation in some places where a traditional text would not. However, there are many very traditional examples, arranged in a reasonable progression of difficulty. I suggest you just skip the examples that are not to your taste. Also, remember that it is Chapter 6, that deals with expectations and variances, so you might want to browse Chapters 1-5 to make sure you have the appropriate background material in mind.