having trouble figuring out how to update a collection

I have a quiz app that I am building for a wedding this weekend.
I have everything set up with auth and adding users to the db.

The next (and final, I think) step will be to collect the user's email and score when they complete the quiz, then appending that to my "scores" collection in the scoresList document as a part of the string array.

I just started playing with this this weekend, so please forgive my NOOBness

thinking to append each new completed user's information into this array

and concat two values together:
<user.email>_<user.score>

felt like that was easier than having a new document for each user, then trying to sort those