Customizing EF Core tables using EF Core 2.0+

ApplicationDbContext.cs

public class ApplicationDbContext : IdentityDbContext<ApplicationUser>
{
    public DbSet<OrderItem> OrderItems { get; set; }

    public ApplicationDbContext(DbContextOptions<ApplicationDbContext> options) : base(options) { }

    protected override void OnModelCreating(ModelBuilder builder)
    {
        base.OnModelCreating(builder);
        
        // Customizations must go after base.OnModelCreating(builder)

        builder.ApplyConfiguration(new ApplicationUserConfig());
        builder.ApplyConfiguration(new OrderItemConfig());

        // Imagine a ton more customizations
    }
}

ApplicationUserConfig.cs

public class ApplicationUserConfig : IEntityTypeConfiguration<ApplicationUser>
{
    public void Configure(EntityTypeBuilder<ApplicationUser> builder)
    {
        // Override nvarchar(max) with nvarchar(15)
        builder.Property(u => u.PhoneNumber).HasMaxLength(15);

        // Make the default table name of AspNetUsers to Users
        builder.ToTable("Users");
    }
}

OrderItemConfig.cs

public class OrderItemConfig : IEntityTypeConfiguration<OrderItem>
{
    public void Configure(EntityTypeBuilder<OrderItem> builder)
    {
        // Make the default column type datetime over datetime2(for some reason).
        builder.Property(o => o.DateTimeOrdered).HasColumnType("datetime");

        // Make the default value 1 for the Quantity property
        builder.Property(o => o.Quantity).HasDefaultValue(1);

        // Make the Primary Key associated with the property UniqueKey
        builder.HasKey(o => o.UniqueKey);
    }
}