It depends on the smartphone. Some Android smartphones have built-in mini-HDMI ports, so getting the phone’s screen on a TV is a simple matter of using a mini-HDMI-to-standard-HDMI cable. That’s pretty rare, though. Other phones require the use of an adapter in order to use a TV as a monitor. For example, Apple makes Lightning-to-HDMI cables for their iOS devices, and many Android phones support the use of a micro-USB-to-HDMI cable.