✅ ? operator and property assigned to =null! meaning in EF core database context

using System;
using System.Collections.Generic;

namespace Learning.Models;

public partial class Book
{
    public int BookId { get; set; }

    public string Title { get; set; } = null!;

    public string? Author { get; set; }

    public string? Isbn { get; set; }

    public int? PublishedYear { get; set; }
    
    public DateTime Date { get; set; }
    public string BookVersion { get; set; }

    public virtual ICollection<Rental> Rentals { get; set; } = new List<Rental>();

    public override string ToString()
    {
        return $"Title: {Title}, Author: {Author}, ISBN: {Isbn}";
    }
}