Im of a “hill of beans” mindset on this.

Fun experiment: take your turntable out, place in a large plate with pieces of the shittiest American cheese slices. We are talking kraft single serves. Splay them out, no deeper than one layer thick is needed, but make a full cover of the microwave bed (on top of the plate though. You will make a mess if not). Then you simply cook the shit out of it. You will see the cheese bubble and burn in the microwave wave peaks and you will see cold cheese in the wave troughs.

I’ve seen the best results when I stack or pile food around in a doughnut shape, and center the hole of the donut shape on the turntable.

There are resonate standing waves inside the microwave. A microwave works at 2.4 GHz. The wave length of 2.4 GHz is 4.92 inches or just under 12.6 centimeters. This is a sinusoidal wave, so half of that wavelength distance is in the trough and half is in the hump of the waveform. These high frequency photons are bouncing around in a Faraday cage made of metal. Their pattern inside the metal box is fixed. The magnetron is off to one side and emitting the radio light from a fix position. As the waves of light bounce around inside, they tend to align into standing wave patterns. Some of the waves cancel out while others work together to amplify little extra energetic spots.

If part of your food is effectively stationary, like at the turntable center of rotation, there is a chance that a dead spot in the radio light wave pattern will form in that region and will not transfer energy to the food. The more the food is offset, the more it should cross points of radio light. So it is always more effective to offset the dish as much as possible.

The more movement the better. That’s why I only put live food in there /s

The turntable is there because the microwaves bouncing around inside the unit don’t do so completely evenly; there will be spots that don’t get heated as much. Moving the food around inside the unit makes it so one of those cold spots isn’t consistently in one place in the food.

So with that as a context, if a cold spot is at or near the center of the turntable, it’s not going to get moved around in the food when it rotates. So better to put the food away from the center.

In my newish place there is a built in microwave without rotating plate. Do modern microwave ovens use technological innovations to spread out the waves somehow clever or is it just a stupid modern microwave?

This varies by microwave as not all microwaves perform the same way, but in my case, for more even heating there are benefits to placing the food just off-center. When the food is placed in the center it doesn’t heat as evenly, which is to do with standing waves forming inside the microwave, which leads to hot and cold spots.

My guess is that the edge or center is relative to the design of individual units, but after starting using the edge it seems to be more even so I’ve kept with it because the level of effort is basically zero.

Doing lower power for longer has far more impact on even heating if you aren’t making something that needs max power like popcorn.

Rotation is good. It makes sure the food gets bombarded by microwave particles on all sides. If it’s on the edge of the turntable, though, the side facing the table would be colder because the microwaves take more effort to hit it, much like how the moon has a dark side because it’s tidally locked around the Earth.

I think that would damage the CD 🤣

All jokes aside, center while covered.