There are at least two mechanisms the food gets heated by in a microwave oven.
One is vibrating of the polarized water molecules (Official main theory). The electromagnetic radiation (radio waves) from an antenna powered by a device called magnetron located inside the oven, which radiation is basically an electromagnetic field that changes polarity billions of times a second (2.4 billions in this case, to be more precise), makes the water molecules inside the oven spin.
Little is known about how the spinning energy of a polar molecule turns to linear motion energy which is heat. I would assume through knocking out of adjacent molecules by the lobes of rotating ones.
The membrane of a living cell is made by a layer of polar molecules called lipid bilayer. It is two molecules thick and the molecules arrange with the polar end on the opposite sides. There are known effects of a static electric and high frequency electric field (microwaves) on a lipid bilayer.
Though these molecules are polar at one end only, they will also oscillate in the presence of the oscillating electric component of a microwave radiation.
The other is through the more known mechanism of inducing electric currents. Electrons are knocked out of their orbits and start moving around alongside the electric component of the field but also due to the magnetic component (Eddy currents), again, back and forth, with the same frequency. Same goes for the remaining ions orphaned of electrons. Heat is caused by the resistance to the flow to the motion of electrons and by the motion of the ions in the opposite direction.
I would think the second mechanism transfers to food more energy from the antenna inside the oven then the first.
Microwave ovens leak radiation in the order of magnitude of cell phones total emitting power therefore a measurable leaked signal from a microwave oven can reach distances in the order of miles. For this reason there was a need to establish a band of frequencies chosen by the manufacturers that was agreed not to be licensed for communications, just above 2.4 Ghz.
It is also likely that this frequency was chosen by the manufacturers for reason of being most penetrating in biological tissues, for uniform heating.
In is also said that cell phone manufacturers have also chosen this unlicensed band initially not allocated for communications to cut through the bureaucracy of getting a license for each channel though i doubt they would have had problems with FCC or other regulating authorities. They don't use the band continuously, but divided it into a number of fixed frequencies called channels, that are in the range of hundreds.
I have made too many times the mistake of assuming that people know what i already knew at the beginning of doing a research or explaining something new. That is since the time i took a course of Inventics in college. When doing the paperwork for an invention, you have to concentrate to demonstrate the new things you bring in based on the most advanced already existing knowledge in that field and not start from basics. Also because an invention is usually addressed to specialists hired by the patenting authority, the so called examiners.
A cell phone is a two way radio device, the same as a walkie talkie. There are differences though. The communication is not done between two phones directly but with the help of one or two towers. Once a call is being made, the network dispatches the call to another tower nearest to the destination or to a landline.
The channels or frequencies (there are always two, emitting and receiving, of full duplex) are being chosen by negotiations with the tower, after "listening" at both ends and finding a free or noise free channel. The channels and even the towers can be switched automatically during the conversation (if by example a channel becomes noisy or the phone is moving like being in a car) without the user noticing. It is said though i doubt it this technology called frequency hopping was co-invented by Austrian born Hollywood actress Hedy Lamarr, once called "the most beautiful woman on screen".
The signal emitted by the cell phone which again is in the range of 2.4 GHz, overlapping the unlicensed band used by microwave ovens has to beat the distance to the tower. However about half of the signal is absorbed by your body, head first.
The phone has a number of automatic emitting power settings, between 0.3 and 3 Watts that are used according to the distance to the tower shown by the number of bars of signal. The difference between first and last is big, and 3 watts is about 3 times the amount of leaked power from a microwave oven (those vary according to the amount of food heated and the position on plate).
3 to 4 minutes is enough to heat most foods the size of a small brain from frozen to boiling in a medium size microwave of 1000 watts full power.
The exposure from a cell phone can be 300 times less than the power needed for heating frozen food inside the oven but the time of exposure to a cell phone radiation during a day can be up to 20 times more for one hour and now people are checking social media or listening to music, that can be hours a day giving a total daily exposure comparable to that of heating frozen food in the oven in 3-4 minutes.
And yes, that exposure is too low to boil your cells (only one case heard of), only enough to re or disorganize the lipid bylayer, disrupt enzymatic processes, damage DNA, etc.
But the heating is not even. There are organs like pancreas that are full of electrolytes (sodium carbonate), stomach (hydrochloric acid), blood vessels (salt) etc.. And it's not only heating. Some lengthy nerves axons are also conductive.
Think about. While you still can.
Now i understand people can't do without them anymore. However there are easy doable ways to avoid most of it. One is using the bluetooth headsets that theoretically uses about 1000 times less power than a cell phone though if you asked me i would say that like in the case of exposure to lead, there are no safe levels.
At home and/or work you can put the phone at least 2 meters away and use it on bluetooth. You can use other devices for checking social media, like a laptop or a tablet which of course need to be connected to the router through a cable rather than wi-fi which has an emitting power comparable to cell phones (and yes in the same unlicensed penetrating band).
Ultimately can consider getting back to a landline and use it with a chorded receiver.
Also it is easy to get rid of most wi-fi emitters in your home, that are of comparable emitting powers with the phones, like the TVs by using the built in ethernet plugs and buying a 10 dollars cable. You will also get better speed by switching to those.
The differences in emitting power of different brands of phones and operating systems (Android, Apple) are minor, they all have to beat the distance to the towers.
The biological long term effects of low level microwave radiation, like from any radiation, are cumulative throughout one's lifetime, which means once enough number of capillaries, small blood vessels and cells are destroyed your body can not recover from that anymore, due to lack of oxygen and/or nutrients at that tissue level.
Frequent, painful, hard to heal infections that lead to so called chronic illnesses including pancreatitis, need to replace articulations, like hip replacement, neurological problems like balance (dizziness), heart problems like arrhythmia, depression and dementia.
