How to spy on a cell phone for free?

To spy on someone’s phone without them knowing became accessible to anyone on the Internet, provided they use spyware. It is a tracker that has to be installed on the target device. But the further process of spying happens online from any user’s device and anytime. The only condition is the Internet availability. How can’t others see that they are being spied on?
Spy applications run in the background and stay unnoticeable. Its icon may and may not be visible on the home screen (it’s optional). This approach guarantees that users can spy on someone’s phone without them knowing.