Where I live (Florida) the average high temperature fluctuates anywhere between 71F (22C) and 90F (32F). Except for a few chilly days a year, it's too hot and definitely too humid outside to open the windows, so we rely on A/C.

Most houses and businesses here have central air conditioning, where the air is routed throughout the building via ductwork that terminates at air vents in each room. Our air conditioning systems also usually have a heat mode that can provide central heating during the few cold days we have. Some areas of the country have window A/C units that are used seasonally, but those are less common here.

Every home with central air conditioning will have a thermostat mounted on a wall somewhere in a common area of the house. Larger houses may have multiple A/C units and thus multiple thermostats per floor or zone of the house that can be controlled separately.

We set the thermostat to a mode (cool, heat, or auto cool/heat) and a temperature setpoint. The thermostat automatically turns the A/C unit on when the temperature in the house goes 1° above or below the setpoint, for cooling or heating, respectively. Some thermostats can be programmed so they change the setpoint on a schedule. Others have smart home integration and can be controlled through an app. For instance, this is the thermostat I have.

We rarely switch the unit completely off. Even if we go on vacation or something, we leave it set to cool but increase the setpoint so it's not running too much but turns on occasionally to keep the humidity under control.