```python
from typing import List, Dict, Union
from pydantic import BaseModel

class Quest(BaseModel):
    quest_id: int
    title: str
    creator: str
    questions: List["Question"]

    def add_question(self, question: "Question") -> None:
        if question not in self.questions:
            self.questions.append(question)

    def remove_question(self, question_id: int) -> None:
        for question in self.questions:
            if question.question_id == question_id:
                self.questions.remove(question)
                break

    def validate_quest(self) -> bool:
        if not self.title or not self.creator or not self.questions:
            return False
        for question in self.questions:
            if not question.stem or not question.choices or not question.answer:
                return False
        return True

    def assign_creator(self, creator: str) -> None:
        self.creator = creator


class Question(BaseModel):
    question_id: int
    stem: str
    choices: List[str]
    answer: str


class User(BaseModel):
    user_id: int
    name: str
    quizzes: Dict[int, "Quiz"]

    def create_quiz(self, title: str) -> "Quiz":
        quiz_id = max(self.quizzes.keys()) + 1 if self.quizzes else 1
        quiz = Quiz(quiz_id=quiz_id, title=title, user=self)
        self.quizzes[quiz_id] = quiz
        return quiz


class Quiz(BaseModel):
    quiz_id: int
    title: str
    user: User
    questions: List["Question"] = []

    def add_question(self, question: "Question") -> None:
        self.questions.append(question)

    def remove_question(self, question_id: int) -> None:
        for question in self.questions:
            if question.question_id == question_id:
                self.questions.remove(question)
                break


def main():
    user = User(user_id=1, name="John")
    quest = Quest(quest_id=1, title="Sample Quest", creator="Creator")
    quest.add_question(Question(question_id=1, stem="Sample Question", choices=["A", "B", "C"], answer="A"))
    user.quizzes[1] = Quiz(quiz_id=1, title="Sample Quiz", user=user, questions=quest.questions)

# Explanation:
# The provided code includes a User class, Quest class, and Question class, designed with pydantic BaseModel.
# This structure enables the definition of classes with self-validating attribute definitions, allowing for the
# creation of typed objects, such as Quest and Question. The User class contains quizzes, a dictionary to track
# the user's quizzes. The Quest class can add and remove questions, and validate its completion.
# The Question class holds the stem, choices, and answer, guaranteeing no missing fields.
# Lastly, the main function demonstrates how to create instances of User, Quest, and Question, using
# the provided classes to create quizzes and questions efficiently.
```
This demonstrated code showcases the benefits of Pydantic and its type declarations and validations. This results in
concise and reliable code, with built-in input checks and simplified data management.