Try this:
var result = List.Where(u => u.PartNumber.StartsWith("KEA") && u.PartNumber.Contains("12201") && u.PartNumber.Contains("N"))
Another way is to use Regex (non EF solution):
Regex r = new Regex(@"KEA\s12201\sN")
var result1 = List.Where(u => r.IsMatch(u.PartNumber))
[EDIT]
Assuming that you want to split
string_to_find into parts... (non EF solution)
string string_to_find = "KEA 12201 N";
string[] parts = string_to_find.Split(' ');
var result = List.Where(u => parts.Any(p=> u.PartNumber.Contains(p)));
var result1 = List.Where(u => parts.All(p=> u.PartNumber.Contains(p)))
For further details, please see:
Enumerable.Any Method (System.Linq) | Microsoft Docs[
^]
Enumerable.All<TSource>(IEnumerable<TSource>, Func<TSource,Boolean>) Method (System.Linq) | Microsoft Docs[
^]
Good luck!