Answer
There are two workable approaches, depending on scope. For just one or two fields, create a reference-type custom field on the target entity (set the type to Read Only, with a Select, Reference, or Expression implementation) that looks up the value from the related entity directly — this is the simpler option when you only need a small number of fields. For linking larger amounts of data (like adding related fields to a list, not just a single field on a form), instead link the two entities together via Query Designer and add that query link into a Projection Configuration, connecting both entities within the projection itself — this is more setup but scales better than creating many individual reference custom fields.