what's the proper way to use request_loader in flask-login

I would like to have the following authentication scheme

• a login page where the user login with email and password, the flask server then returns a JWT token
• for later API access, the user has to provide the JWT token in the authorization header

However, I don't fully understand the inner working of flask-login. My preliminary understanding is that

• After user submits the password, we call login_user and flask-login creates a session to store user information (maybe user_id?)
• Next time a request comes, authentication is done before processing the request if login_required decorator is used

I thought only request_loader would be needed if JWT authentication is used because all we need to do is check every request header. But if only the request_loader function is provided (see code below), exception is thrown.

Exception: No user_loader has been installed for this LoginManager. Add one with the 'LoginManager.user_loader' decorator.

More specifically, my questions are

1. what are the roles played by user_loader and request_loader if we are to use JWT authentication?
2. when are user_loader and request_loader being called, if both of them are provided?
3. why do we still need user_loader if request_loader is provided?

Here are my implementation of these two loaders

@login_manager.user_loader
def load_user(user_id):
    return User.query.get(int(user_id))

@login_manager.request_loader
def load_user_from_request(request):

    auth_str = request.headers.get('Authorization')
    token = auth_str.split(' ')[1] if auth_str else ''
    if token:
        user_id = User.decode_token(token)
        user = User.query.get(int(user_id))
        if user:
            return user
    return None