Nearly always when a remote control doesn’t work, the underlying problem is elementary: the unit does not emit light. The cause may be dry solder joints, defective LEDs etc., but also a flat battery (perhaps due to the stuck key).
The human eye is unable to perceive infra-red light. By contrast, an ordinary phototransistor like the BP103 has no problems working in the infrared spectrum, so in the circuit here it simply biases the BC558 which, in turn, makes LED D1 flash in sympathy with the telegram from the remote control. The preset in the circuit determines the sensitivity.