Ticket fields are the items within a ticket form that contains various metadata for the ticket.
Instead of managing Zendesk ticket fields via Zendesk itself, we instead use GitLab itself via the ticket-forms-and-fields project. This allows us to have version-controlled ticket fields. For more information on managing Zendesk via the various GitLab projects, please review Using the sync repos.
As we have many fields, and they change quite frequently, the best resource to see all the current fields would be the ticket-forms-and-fields project.