Which model will select to be base of view sales order entry form ( salesfooter model or sales header model) ?

Take a closer look at your models, especially at underlined lines:

Sales Header Model

public int SalesOrderNo { get; set; }  //1
public int SalesYear { get; set; }
public int BranchCode { get; set; }
public int SalesType { get; set; } //2
public DateTime SalesDate { get; set; }
public int CustomerID { get; set; }
public Customer Customers { get; set; }
public ICollection<SalesFooter> SalesFooters { get; set; } //3

Sales Footer model

public int SalesOrderNo { get; set; } //1
public int SalesLineNo { get; set; }
public int SalesType { get; set; } //2
public int BranchCode { get; set; }
public string ItemCode { get; set; }
public decimal Quantity { get; set; }
public decimal UnitPrice { get; set; }
public decimal Total { get; set; }
public Items Itemes { get; set; }
public SalesHeader SaleHeaders { get; set; } //3

1. SalesOrder is common part of both models.

2. SalesType, similary to SalesOrder, is common part of both models.

3. Collections of SalesHeader and SalesFooter are related to each other.

This sounds like bad design!

In your case, you need to create one-to-many relationship, where

SalesOrder         SalesOrderDetails
SalesOrderNo (PK) 1 -> ∞ SalesOrderNo (FK)

* PK - Primary Key
* FK - Foreign Key

Let's illustrate it on real data:

SalesOrders (SalesHeader)
SalesOrderNO   1
SalesYear      2019
SalesDate      2019-01-17
BranchCode     1
SalesTypeNo    1
CustomerNo     2509

SalesOrderDetails (SalesFooter)
SalesOrderNo SalesLineNo ItemCode  Quantity  Price Total
1            1            12929       5        10   50
1            2            17918       4         5   20

So, rewrite your models to mirror above data. Note, that SalesOrderNo field you can hide in view.

Please, read more about Relational database[^]

[EDIT]
Example:

void Main()
{
	
	List<SalesOrder> orders = new List<SalesOrder>()
		{
			new SalesOrder(){SalesOrderNo = 1, SalesDate = new DateTime(2019,1,17), BranchCode = 1, SalesTypeNo = 1, CustomerNo = 2509},
			new SalesOrder(){SalesOrderNo = 2, SalesDate = new DateTime(2019,1,17), BranchCode = 2, SalesTypeNo = 2, CustomerNo = 3945}
		};
		
	List<SalesOrderDetails> details = new List<SalesOrderDetails>()
		{
			new SalesOrderDetails(){SalesDetailNo = 1, SalesOrderNo = 1, SalesLineNo = 1, ItemCode = 12929, Quantity = 5, Price = 10, Total = 50},
			new SalesOrderDetails(){SalesDetailNo = 2, SalesOrderNo = 1, SalesLineNo = 2, ItemCode = 17918, Quantity = 4, Price = 5, Total = 20},
			new SalesOrderDetails(){SalesDetailNo = 3, SalesOrderNo = 1, SalesLineNo = 3, ItemCode = 13546, Quantity = 5, Price = 6, Total = 30},
			new SalesOrderDetails(){SalesDetailNo = 4, SalesOrderNo = 1, SalesLineNo = 4, ItemCode = 11001, Quantity = 100, Price = 7, Total = 700},
			new SalesOrderDetails(){SalesDetailNo = 5, SalesOrderNo = 2, SalesLineNo = 1, ItemCode = 12828, Quantity = 20, Price = 8, Total = 160},
			new SalesOrderDetails(){SalesDetailNo = 6, SalesOrderNo = 2, SalesLineNo = 2, ItemCode = 15467, Quantity = 5, Price = 12, Total = 60},
			new SalesOrderDetails(){SalesDetailNo = 7, SalesOrderNo = 2, SalesLineNo = 3, ItemCode = 18741, Quantity = 3, Price = 30, Total = 90},
			new SalesOrderDetails(){SalesDetailNo = 8, SalesOrderNo = 2, SalesLineNo = 4, ItemCode = 13698, Quantity = 2, Price = 100, Total = 200},
			new SalesOrderDetails(){SalesDetailNo = 9, SalesOrderNo = 2, SalesLineNo = 5, ItemCode = 12587, Quantity = 8, Price = 8, Total = 64},
		};
	
	foreach(SalesOrder SalesHeader in orders)
	{

		Console.WriteLine("\n{0}{1}{2}", new string('=', 3), " Order ", new string('=', 3));
		Console.WriteLine("SalesOrderNo: {0}\nSalesDate: {1}\nSalesYear: {2}\nBranchCode: {3}\nSalesTypeNo: {4}\nCustomerNo: {5}",
			SalesHeader.SalesOrderNo, SalesHeader.SalesDate, SalesHeader.SalesYear, SalesHeader.BranchCode, SalesHeader.SalesTypeNo, SalesHeader.CustomerNo);
		Console.WriteLine("{0}{1}{2}", new string('=', 3), " Order details ", new string('=', 3));
		List<SalesOrderDetails> items = details.Where(x=>x.SalesOrderNo==SalesHeader.SalesOrderNo).ToList();
		foreach(SalesOrderDetails SalesFooter in items)
			{
			Console.WriteLine("SalesLineNo: {0}\tItemCode: {1}\tQuantity: {2}\tPrice: {3}\tTotal: {4}",
				SalesFooter.SalesLineNo, SalesFooter.ItemCode, SalesFooter.Quantity, SalesFooter.Price, SalesFooter.Total);
			}
	}
	
	
		
	
}

// Define other methods and classes here
public class SalesOrder //SalesHeader
{
	public int SalesOrderNo {get; set;}
	public DateTime SalesDate  {get; set;}
	public int SalesYear  {get=>SalesDate.Year;}
	public int BranchCode  {get; set;}
	public int SalesTypeNo  {get; set;}
	public int CustomerNo  {get; set;}
}

public class SalesOrderDetails // SalesFooter
{
	public int SalesDetailNo {get; set;}
	public int SalesOrderNo {get; set;}
	public int SalesLineNo {get; set;}
	public int ItemCode {get; set;}
	public int Quantity {get; set;}
	public decimal Price {get; set;}
	public decimal Total  {get; set;}	
}

Result:

=== Order ===
SalesOrderNo: 1
SalesDate: 2019-01-17 00:00:00
SalesYear: 2019
BranchCode: 1
SalesTypeNo: 1
CustomerNo: 2509
=== Order details ===
SalesLineNo: 1  ItemCode: 12929  Quantity: 5  Price: 10  Total: 50
SalesLineNo: 2  ItemCode: 17918  Quantity: 4  Price: 5  Total: 20
SalesLineNo: 3  ItemCode: 13546  Quantity: 5  Price: 6  Total: 30
SalesLineNo: 4  ItemCode: 11001  Quantity: 100  Price: 7  Total: 700

=== Order ===
SalesOrderNo: 2
SalesDate: 2019-01-17 00:00:00
SalesYear: 2019
BranchCode: 2
SalesTypeNo: 2
CustomerNo: 3945
=== Order details ===
SalesLineNo: 1  ItemCode: 12828  Quantity: 20  Price: 8  Total: 160
SalesLineNo: 2  ItemCode: 15467  Quantity: 5  Price: 12  Total: 60
SalesLineNo: 3  ItemCode: 18741  Quantity: 3  Price: 30  Total: 90
SalesLineNo: 4  ItemCode: 13698  Quantity: 2  Price: 100  Total: 200
SalesLineNo: 5  ItemCode: 12587  Quantity: 8  Price: 8  Total: 64

Good luck!