Under the hood, each entity actually does have it's own id in order to maintain uniqueness. The same applies to associations, which are actually tracked in relationship tables within the actual database, so functionally, associations will cover the bases of foreign keys.
The domain model is more of an abstracted view as it's more visual for the developer. So in the modeler, the dev doesn't need to worry about primary/foreign keys while developing since the Mendix runtime handles the mapping of the objects in the modeler to the database schema.
thank you very much for you answer, have good times, friend
In addition to Hardy’s useful answer.
Domain model is a business domain model, so the business and citizen developers need to understand it easily. It is not a database model. But you are able to set validation constraints and indexes for data integrity and performance.