Once students take the assessment, check their results, and select Finish, they will then be taken to the differentiation feature (in this case a YouTube link). As the teacher you can see the results of the assessment as well as who has said they watched the video and who did not.
Embedded below is a 3 minute and 42 second YouTube tutorial.
For a blog post I created about how to create an assessment in Quizizz - click here. Please note it was made 2 years ago.