AM radio multiplexing question: transmitting SSB in sync with AM (numbers
stations, clandestine broadcasting)

I understand that it is possible to multiplex SSB transmissions on top of AM
transmissions, especially on SW.
The AM transmission is decoded by the envelope or PLL coherer, the SSB
transmission is ignored.

On a properly tuned SSB receiver, the AM transmission only partially
interferes with the SSB transmission.

Some numbers stations use this transmission technique, but it just as valid
if say Radio Free Asia were to use this technique by transmitting over
Chinese domestic radio.

1. What is the math behind this transmission working (SSB is a form of Angle
Modulation, not Amplitude Modulation)?

2. Are some receivers more affected by (interference where) SSB
transmissions superimposed over AM than others? (PLL versus SYNC vs Envelope
Detectors)