By rectification and filtering of RF signals✓Bonne réponse
By breakdown of the Zener voltage
By mixing signals with noise in the transition region of the diode
By sensing the change of reactance in the diode with respect to frequency
Explication
A diode envelope detector rectifies the modulated RF so only one polarity remains, and an RC network then filters out the RF component while following the slower envelope. That envelope is the recovered AM audio.