Low frequency receiving scheme/circuit
The input clamp protects the downstream amplifiers, in the case when the keyfob is too close to the transmitter.

Dynamic range of these keyfob 125KHz data links is 100,000X in voltage (100dB).

Regarding the rectifiers: diodes can be used (in which case you likely need to buffer, to provide the diode current); other active (uses power) circuits use transistors to "rectify" and can be much more sensitive......but need current to operate.

Why FOUR amplify/rectify behaviors? to implement an accurate RSSI --- relative signal strength indicator, even as the signal varies 10,000:1 or 1,000:1.

No idea about the 129KHz.