Not understanding why I can call interface methods without concrete implementation

Question

I am working on a Spring Boot application which uses Spring Data Jpa. I cannot give you the exact code since I am not allowed to, but I can simplify my issue so you may get an idea:

@Query("SELECT u FROM User WHERE u.uuid = :#{#uuid}")
Optional<UserInterface> findProjectedUserByUuid(UUID uuid)

This Query return an interface of type UserInterface:

public interface UserInterface{
    String getName();
    String getEmail();
}

Somewhere in my service class I get to this point:

UserInterface userInterface = userRepository.findProjectedUserByUuid(uuid).get();

System.out.println(userInterface.getName());

The application correctly prints out the name of the user.

My question is: why can I call userInterface.getName() in the code and return the correct name. Isn't this supposed to be an interface? How is it holding data?

My guess is...but I am not really sure about this, hence why I am asking you guys, this is a case of polymorphism. I believe that the repository is actually returning a class which implements my interface? And maybe that is why I am able to call methods on it. But if this is true, is there any way to see which class gets returned or use it in any meaningful way?