List<string> testList = new List<string>(){"123","456","789","123","456","789","123","456","789","123","456","789","123","456","789"};

Lambda	
List<string> test = testList.Where(v => v == "123").ToList();

Linq
List<string> test = (from x in testList
    where x == "123"
    select x).ToList();

foreach (var num in test)
{
     Console.WriteLine(num);
}
            
Console.ReadLine();

Results:
"123"
"123"
"123"
"123"
"123"

List<string> testList = new List<string>(){"123","456","789","123","456","789","123","456","789","123","456","789","123","456","789"};
	
List<string> test = testList.Where(v => v == "123").ToList();
Console.WriteLine(test);




Results:
A List<string> with 5 "123".

These are statements to pull all the matching values (123) from the List.

Lambda and Linq Statements to pull all matching values from List

C#