1. On AM the radio energy literally pulses (the amplitude modulation) and thus the arcs of plasma will pulse too thus creating the audio noise. AM has a carrier wave that’s constant but the two sidebands of signal pulse from zero energy at silence to more power the louder the sound being transmitted. FM signals broadcast essentially the same power all the time since it’s the frequency and not the amplitude that’s changing.
2. Because the signal frequency is much lower on the AM broadcast bands the wavelength is much larger and thus the antennas are much bigger. On AM the tower itself is typically the antenna vs FM radio where the antenna is typically only a meter or two long at the top of a very tall tower. That’s what makes AM towers more dangerous as the tower can be carrying many kW of energy and if you touch it you’ll go zap zap. The towers typically sit on top of a ceramic insulator to insulate them from the ground whereas FM towers typically just are attached right to the ground (although with grounding straps for lightning protection).
Finally (some folks don’t always know this) you can operate AM on the frequencies typically used for FM… it’s just a mode and works on any frequency. Aviation radios operate on AM but in the VHF band near FM broadcast frequencies.