var client = new MongoClient("mongodb://localhost:27017");

var database = client.GetDatabase("db224");
database.DropCollection("customers");

var collection = database.GetCollection<Customer>("customers");
collection.InsertMany([
    new() { Id = 1, City = "London", Country = "UK" },
    new() { Id = 2, City = "London", Country = "USA" },
    new() { Id = 3, City = "Seattle", Country = "UK" },
    new() { Id = 4, City = "Seattle", Country = "USA" }
]);

var results = collection.AsQueryable()
    .Where(c => c.City.IndexOf("e") != -1)
    .ToList();

foreach (var result in results)
{
    Console.WriteLine($"Found {result} {result.City} {result.Country}");
}

public class Customer
{
    public int Id { get; set; }
    public string City { get; set; }
    public string Country { get; set; }
}