Keycloak Userstorage SPI: why both the getUserById() and getUserByUsername() called when I request for tokens?

I implemented a Userstorage SPI for Keycloak and I see that when I try to have my tokens, it uses both the getUserByUsername() and getUserById() method (https://www.keycloak.org/docs-api/13.0/javadocs/org/keycloak/storage/user/UserLookupProvider.html):

1. I try to have my tokens like this:

I implemented the methods here (I know the logging message is not the best, this is just a tutorial project):

public class UserStorageProviderImpl implements UserStorageProvider, UserLookupProvider, CredentialInputValidator, UserQueryProvider {
   
   ...

    @Override
    public UserModel getUserByUsername(String username, RealmModel realm) {
        log.info("------------------- username = " + username);
        User user = UserRepository.findByName(username, realm, model, session);

        AbstractUserAdapterImpl um = Mapper.toUserModel(session, realm, model, user);

        return um;
    }


    private static final Logger log = LoggerFactory.getLogger(UserStorageProviderImpl.class);

    @Override
    public UserModel getUserById(String id, RealmModel realm) {
        log.info("------------------- id = " + id);
        StorageId sid = new StorageId(id);
        String name = sid.getExternalId();
        User user = UserRepository.findByName(name, realm, model, session);
        return Mapper.toUserModel(session, realm, model, user);
    }
   
}

As you can see, both log messages can be seen (------------------ username = and -------------------- id = )

My question is: why? Sadly I did not find any information in the documents about the flow it works. How each methods following eachother.