This security alarm works by NE555 timers and EXOR logic gates. When the activation selector is open the circuit does not work because the IC2 output is high, then the Q12 collector turns off the Q3-Q4 transistors.
When the selector gets closed the IC2 output switches off after a time given by the C5 capacitor, so the security alarm starts working. If a sensor changes condition, an EXOR logic gate switches on for a bit.
The IC1 timer turns on, then the IC3 timer starts counting. If the circuit is not deactivated, the IC3 output triggers the alarm sound, which is stopped by to the IC4 timer.
The circuit in the video works with a small time delay, because small C3-C4 capacitors have been used. By using the values indicated in the circuit diagram the security alarm should work with a time delay of about 10-15 seconds.