The term RF "amplitude modulation" is a misnomer.

I don't have an in-depth knowledge of radio communication. I only know these basic.

We have music or speech as input to the microphone. This input is variation in air pressure making sound waves. The transmitter circuit has the ability to produce an electrical signal waveform with a time series of voltage/time (this electrical signal waveform is an "analogous waving", waving in voltage imitating the waving of the sound - thus the term "analog" signal). Now the radio station has a fixed radio frequency to broadcast, say f Hz. Assume we take a pure sinusoidal voltage waveform with frequency f. We superimpose the analog signal onto this base waveform producing a amplitude modulated sine waveform with frequency f.

The transmitter somehow is connected to some resistor - the antenna. This antenna will emit em wave with the same frequency f. Users with radio tuning in to this frequency f will get the broadcast of the radio station.

Now, we have the term "amplitude modulation" to describe this manner of radio transmission. It is assumed that the electric/magnetic fields E/M have undergone modulation in their amplitude. We assume the em waves of frequency f has the same "analogous" waveform as the electrical signal in the transmission antenna. The only difference is that now it is the E/M fields that are varying instead of voltage/current in the antenna. But this picture of amplitude modulation may just be an illusion, not reality.

We have the picture of our radio wave having an electric field E varying in space, but this E field is only from our Maxwell model of electromagnetic waves. But no one has ever measured such E fields nor how they vary in strength. It is all just from our electromagnetism theory - basically Maxwell's.

If we accept the photon theory of light, then the radio waves are just photon. So the "amplitude variation" is just variation in energy of the photon and E-photon = hν; h= Planck constant. As frequency f is fixed, what is modulated is the photon energy E-photon.

So I'll rather say "amplitude modulation" may be a misnomer. More rightly it should be termed "RF intensity modulation".