Add additional fields in Drupal 8 node edit form
You can add fields to nodes both utilizing the UI and programatically.

To do it over the interface, go Structure > Content Types (in the admin menu) - you will see a list of all node types currently available to your site. Find the node type you want to add fields to and hit 'manage fields' (preselected in the dropdown button to the right).

Now you can use 'add field' to add fields of any type you have enabled/downloaded from drupal. (If you don't have a whole lot of field types to select from, check if their modules are enabled under 'Extend'.)