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How to configure many to many relationship using entity framework fluent API

modelBuilder.Entity<Account>()
            .HasMany(a => a.Products)
            .WithMany()
            .Map(x =>
            {
                x.MapLeftKey("Account_Id");
                x.MapRightKey("Product_Id");
                x.ToTable("AccountProducts");
            });

ManyToMany Relation in EF Core fluent API

You need to use some of the other UsingEntity overloads which allow you to configure the left and right navigations.

For instance

entityTypeBuilder1
    .HasMany(lambdaManyFirst)
    .WithMany(lambdaManySecond)
    .UsingEntity<Dictionary<string, object>>(joinEntityName,
        j => j.HasOne<TEntity2>().WithMany().HasForeignKey(fkName2),
        j => j.HasOne<TEntity1>().WithMany().HasForeignKey(fkName1),
        j => j.ToTable(joinTableName)
    );

How Configure one to Many Relationship From AspNetUsers Table To ... using Fluent API

add relations

public class AppUser : IdentityUser
    {
        public string DisplayName { get; set; }
        public string Bio { get; set; }

         public virtual  ICollection<Sale> Sales { get; set; }
    }
}

    public class Sale
    {
        public int SaleID { get; set; }
        public int SaleAmount { get; set; }

        public int UserID { get; set; }
        public virtual AppUser User { get; set; }

    }

Since you are using net core 5 you don't need any fluent apis in this case

but if you use another version you can add

modelBuilder.Entity<Sale>(entity =>
            {
                entity.HasOne(d => d.User)
                   .WithMany(p => p.Sales )
                   .HasForeignKey(d => d.UserId);
            });

How to use Fluent API for creating a simple one-to-many relationship

The simple answer to your last question. EF is recognizing that Area.Id is a primary key so connects Location.AreaId to Area.Id

Also, here is a simple guide on how to do it.

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