What happens if I don’t finish my survey and click on Finish? It also does not contain mandatory questions, but the survey has been marked as mandatory

Surveys are only marked as mandatory so that they can be classified as Expired, Pending, Completed. If you only half-answer a survey that is marked mandatory and click on Finish, the action will go through. The downside of this is that you will not be able to take the survey again. It will have to be assigned to you again by the survey creator.