The crystal frequency isn't shown. I'm guessing 10 MHz and my answers are based on that.
1. The output of the CD4068 will be on for a very small fraction of the time. About 0.1 ms every 25 ms. That will initiate the pulses of sound.
2. CD4060 Q7 oscillates at 78 kHz. The transmitter drive is therefore 78 kHz for about 0.1 ms every 25 ms.
3. While the pulse is happening, C3 charges up via D1. After the pulse finishes, C3 discharges via RV1 and R2. Once that voltage is low enough, the output on N1 will stop the N2/N3 latch operating. So RV1 varies the maximum time, and therefore the maximum distance, at which an object can be sensed.
4. A1 is an amplifier to create a mid-rail voltage, so that the other amplifiers can have a positive or negative signal compared to the mid voltage.
T1 is to turn off the receiver while the transmit pulse is happening.