T1 is an amplifier stage.
The detector stage:
VR (1M0) is adjusted so that without an input signal (beam broken) the collector of T2 is near 0V. (T2 is turned ON)
With no input signal (broken beam) the base of T3 is OV so it is turned OFF. The LED has no voltage across it so it is also OFF. T4 gets a base current through R7 and is turned ON. With T4 turned ON the buzzer sounds.
When the beam isn't broken there is an AC signal (through C2) into the base of T2. This causes it to switch ON and OFF at the ultrasonic frequency. D1 allows the positive half cycle to charge up C3 and C4 (through R5). This produces enough (steady DC) current to turn on the LED and T3. (although though I'm not convinced that the LED is in the correct place as the base emitter junction would clamp the voltage to about 0.7V - way below the voltage need for the LED )
With T3 turned ON, T4 is turned OFF so the buzzer doesn't sound.
Proposed modification to existing circuit: - improved LED indicator
I'm not happy with the position of the indicator LED so by adding an extra NPN transistor (as a simple switch) when the voltage across C3 is high (beam not broken) T5 turns ON and the LED is lit. You can use any colour LED.
When the beam is broken C3 decays rapidly, T5 and T3 are switched OFF
The 100k resistor is there to discharge the capacitors and to make sure T3 is fully OFF (pull down). I don't see C4 as being necessary.