Microwaves penetrate food to a depth of generally 1 to 1.5 inches. The rest of the food mass heats via conduction. Thus the full mass of a slice of bacon or pizza gets heated ... but a roast or casserole involves some surface-to-interior conduction. Microwave energy hits the food surfaces from all directions, including both sides and top at corners of a square or rectangular dish. It's sometimes advised to shield the corners of a dish with pieces of aluminum foil to avoid overcooking those areas (brownies, cake, etc.).
Microwaves aren't sufficiently energetic to break chemical bonds, ionize molecules, or damage DNA. Heat is generated by inducing vibration/motion of dipolar molecules (aligning with the oscillating magnetic field), primarily water but also some mild effect on fats and sugar. Bodily injury can be caused by the heating, not by ionization or damage to DNA. The lens and cornea of the eye are particularly vulnerable to injury from heat damage that can cause a cataract effect. I've read advisories to always pause a running microwave oven before opening it to avoid even momentary exposure to the microwave energy, although it travels at literally the speed of light so seems any residual energy after the door interlocks trigger would be absorbed by the food before the door is open enough for any of it to escape.
Microwaves are a frequency range of 300 Mhz to 300 Ghz on the electromagnetic spectrum, between radio waves and infrared. Microwave ovens operate on a frequency of 2.45 Ghz. Infrared, visible light, ultraviolet, x-ray, and gamma rays are higher on the spectrum.
Microwaves are sometimes considered a type of radio wave and can be focused into a beam for use in point-to-point communications. Satellite TV systems, WiFi, Bluetooth, and cell phones all operate within the microwave range of frequencies.
