How to access fields for the Registrant settings (RNG)
But how can I access these fields?


As I'm sure you are aware, you can add the fields here at

Administration &gt; Configuration &gt; RNG &gt; Registrant settings &gt; Manage fields (/admin/config/rng/registrant/fields)


  I can see my registrants on the Registration List, but I can't see the additional fields.


The registration list page is simply a page provided by Views. There are a few ways to edit the view, including the quick edit button which appears on the right side of the page. On the view, there is already a pre-made relationship from event entity -&gt; registrant entities -&gt; identity entity. The fields added to any of the entity types, including the registrant entity type, should be automatically available by using the standard Views add field button.


  I also have no idea how to add the content to these fields?


Before of 1.2, there was no edit registrant entity form (or view registrant entity). Adding field values programmatically to registrant entities is also fully supported using the standard Drupal entity API.

I have created an issue for RNG which can be used to discuss this kind of feature: Add a way to view and edit registrants entities