how to create multiselect dropdownlist of checkbox

FIrstly, You create a Model.
Like I have an example- You have a product with Id, name, and  selected for track the state.

public class Product
{
    public int Id { get; set; }
    public string Name { get; set; }
    public bool Selected { get; set; }
}

In a Controller create a list of products and you also used your databae if existed.

public IActionResult Index()
{
    List products = new List
    {
        new Product { Id = 1, Name = "Product A" },
        new Product { Id = 2, Name = "Product B" },
        new Product { Id = 3, Name = "Product C" }
    };

    return View(products);
}

Create a view, design depend on requirement where you will use html tag helpers, css also javascript for best user experience. All depends on your choice. for example, given a simple view code

@model List


    @for (var i = 0; i < Model.Count; i++)
    {
        

            
            @Model[i].Name
            
        
    }
    Submit

As your ques here I used checkbox. And at the end in your controller add the HTTP POST method to handle the response (for process the selected item)

[HttpPost]
public IActionResult Index(List products)
{
    var selectedProducts = products.Where(p => p.Selected).ToList();

    // Perform your actions with the selected products

    return RedirectToAction("Index");
}