How do I link fields of a 'user content type' to the actual logged in user? I'm using the User Content Type module (Drupal 7) in order to store some fields for each user, such as "zip code."

Now, in this node_edit page, I made a variant for a Catalog node, for a specific role. (Let's say "salesman".)

How do I link the actual logged in user with their respective User node, to show and store the "zip code" field in the Catalog node?
In Drupal 7, the simplest way to associate fields with a specific user is via Admin => Configuration => People => Account settings => Manage fields.

Because of its simplicity, I recommend this approach as a starting point. If it's not flexible enough, consider the contributed Profile 2 module.