Java Spring MongoDB Repository Interface

So I have an AppUser class:

    @Data
    @Builder
    @Document(collection = "app_users")
    @Component
    @AllArgsConstructor
    @NoArgsConstructor
    @Import(AppConfig.class)
    public class AppUser {
    
      @Id
      @NotBlank(message = ErrorConstants.ANDROID_USER_ACCOUNT_MANAGER_ID_IS_NULL)
      private String androidUserAccountManagerId;
    
      @NotBlank(message = ErrorConstants.NULL_NAME)
      private String name;
    
      private Friend bestFriend;
    
      @Setter(AccessLevel.NONE)
      private FriendList friendList;
    
      private boolean manualBestFriendOverride;
    
      public Optional<Friend> getFriend(String friendName) {
        return friendList.getFriend(friendName);
      }
    
      public void calculateBestFriend() {
        if (!manualBestFriendOverride) {
          bestFriend = friendList.calculateAndReturnBestFriend();
        }
      }
    }

I have created an AppUserRepository interface that extends MongoRepository:

    @Repository
    public interface AppUserRepository extends MongoRepository<AppUser, String> {}

I have a WebController class that interacts with the interface. The AppUserRepository field in this class is @Autowired. This all seems to work but I have a few questions regarding how, and how I go forward and write integration tests for this:

1. How do I configure this AppUserRepository that has been created? Can I run it on a specific port etc?
2. Why has the Autowiring worked as I have not created this AppUserRepository Bean in an AppConfig like I have other Beans that are Autowired in my application.
3. If I was to create a Bean, wouldn't I also have to implement the class and return the instantiation? I started doing this but I had to implement all of the MongoRepository classes methods which I wasn't sure seemed quite right.
4. How do I write integration tests with an AppUserRepository? I need an AppUserRepository for my requests to interact with, but I do not want this to be the same DB as the real-time application DB when the service is up and running. Can I @Autowire the database into the integration test class and then close the DB after the integration tests run? If this is how I go forward, I think I then need to do point 3 above.

Thanks for your help in advance, I have tried reading some documentation but I think I am missing some key knowledge that means it is all quite overwhelming and confusing.

Thanks!

That's actually quite a big story to tell. This topic is called Spring Data JPA, Hibernate. You might wanna do a research on that, and watch some tutorials and so on.

Briefly, that MongoRepository just gives you a lot of methods which you can use. You can also define your own methods, add queries and etc.

Your starting points: https://www.baeldung.com/spring-boot-hibernate

https://www.baeldung.com/the-persistence-layer-with-spring-data-jpa

https://www.baeldung.com/spring-data-jpa-query

Of course you can set a port number (and some other properties) via application.properties file. This is a list of most common properties, you can find properties for mongodb on it:
https://docs.spring.io/spring-boot/docs/current/reference/html/appendix-application-properties.html

Now about bean. You basically created one with @Repository annotation actually. So Spring Context loads it on the start of application. You can autowire it.