How do I display a field from a referenced user? I have a content type A with a reference to a user and I would like to show a specific field from that user in nodes of that content type.

How can I achieve this?
You might achieve this with Views, by creating a view for nodes of that content type, and then adding a "relationship" to the user. The user fields will then become available for you to display.

You can find an introductory tutorial to start using Relationships with Views in "How relationships work in Views and how to add a referencing entity relationship".