How to create a custom content type with a quiz field?
You can use the Field Quiz module. It already has an alpha version for Drupal 8.

According to the module description,


  This module provides a simple field that can be used as a simple quiz
  or multiple choice questions. It provides a field with a textfield and
  a checkbox / radio selection combined. If added to any entity
  (node/content type, user, taxonomy term, ...) the editor of the entity
  can enter multiple answers in the textfields and determine which
  answers are correct.
  
  This module is for everybody who does not want to use the whole
  complex system of the quiz module, but just a simple field for
  multiple choice questions.