C#: How do you implement dependency injection?

1builder.Services.AddScoped<IUserService, UserService>();
2builder.Services.AddSingleton<ICache, MemoryCache>();
3builder.Services.AddTransient<IEmailSender, SmtpSender>();

1public class OrderService(IUserService userService)
2{
3    public async Task<Order> CreateAsync(CreateOrderDto dto)
4    {
5        var user = await userService.GetByIdAsync(dto.UserId);
6        return new Order(user);
7    }
8}