Can DI be used without a DI container? How?

Yes, DI can be implemented manually without using a DI container. You simply pass

dependencies through constructors or methods:

public class OrderService
{
private readonly IOrderRepository _repo;
public OrderService(IOrderRepository repo)
{
_repo = repo;
}
}

// Manual injection

var repo = new OrderRepository();
var service = new OrderService(repo);

This is suitable for small or simple applications, though not scalable for large apps.