context.AddRange(
    new Customer { CustomerID = "1", City = "abc" },
    new Customer { CustomerID = "2", City = "def" },
    new Customer { CustomerID = "101", City = "ghi" },
    new Customer { CustomerID = "102", City = "jkl" });

context.SaveChanges();

var results = context.Set<Customer>().Where(c => c.City.IndexOf("e") != -1).ToList();

using var context = new SomeDbContext();
context.Database.EnsureDeleted();

context.AddRange(
    new Customer { CustomerID = "1", City = "abc" },
    new Customer { CustomerID = "2", City = "def" },
    new Customer { CustomerID = "101", City = "ghi" },
    new Customer { CustomerID = "102", City = "jkl" });

context.SaveChanges();

var results = context.Set<Customer>().Where(c => c.City.IndexOf("e") != -1).ToList();
Console.WriteLine(results.Count);
foreach (var result in results)
{
    Console.WriteLine($"Found {result} {result.City}");
}

public class SomeDbContext : DbContext
{
    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder) 
        => optionsBuilder
            .UseMongoDB("mongodb://localhost:27017", "SomeDb4")
            .EnableSensitiveDataLogging()
            .LogTo(Console.WriteLine, LogLevel.Information);

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Customer>().ToCollection("Customers");
    }
}

public class Customer
{
    public string CustomerID { get; set; }
    public string City { get; set; }
}