I think you are off to a good start 
When I click on one of the quiz links up top, I see the quiz name followed by two buttons.
I tried clicking on the history button but it didn’t appear to do anything. But also, I wasn’t sure what that was supposed to be. So maybe consider just having one button for start or begin.
Once I started the quiz, I did notice an issue with the answer selection. For example, I am doing the algebra quiz, and say if I selected the second option for question 1, then the next question would also have the second option already selected.
Looking at your code, it looks like the value for the name attribute is being used for all sets of questions. Right now it is name="choice". But each question should have a different name. In your `quiz.js` file , I would add the logic for giving each question a unique name value.
Design has never been my strong suit but my general impression is that there is a lot of whitespace. Whenever, I have to come up with designs, I always study existing sites to see what patterns they are using. So you can try that route 
You could consider grouping some of these files. For example, you have _flex.scss and _space.scss files. In the root directory, you can create a scss folder and follow the suggested guidelines here in the sass docs
It talks about a 7-1 pattern. I don’t know if you need all of that for this project. but it is helpful to learn and maybe implement some of it. 
As for your json files, you could consider putting those inside of a data directory.
As for your quiz-start file, I would consider renaming that to something like quiz-init. That way it is more reflective of what is happening since that logic is for initializing a quiz. you could also consider moving that logic to the quiz.js itself.
Also, that quiz-start logic is starting to get repetitive. Right now you are attaching a click event to each link. Instead, you can have an array of links and quizTypes and then use a forEach to loop through each of the links add attach an event listener.
Hope that helps