A bit of chemistry and why microwaves heat things up.
The water molecule has two hydrogens and one oxygen, they are arranged in a V configuration with the Oxygen being the point of the V. The angle is 109 degrees or thereabouts.
The oxygen end has a higher electronegativity, which basically means it wants the hydrogens electrons a bit more than the hydrogens themselves want them. So the molecule becomes a miniature electric dipole - slightly more negatice at the oxygen end than the hydrogen side.
Now here comes a microwave, tuned to the right frequency.
The “electric” part of the microwave pushes the water molecule.. just like a child on a swing. The more it pushes, the more the water molecule starts to rotate - it literally spins, end over end, and absorbs the energy of the microwaves.
Now this is NOT the classical definition of heat. Heat is molecules bouncing around like pool balls. But the rotational energy of the water molecules causes huge disturbances and soon, things are zipping around and bumping into each other all over the place.
You are correct that microwaves of the correct energy can break apart various bonds. Each substance might require different frequencies, but there is no magic here.
As far as understand it, thats it.
One more neat thing. Ever see frost crystals on windows during the winter? Notice the intricate patterns and sworls that get formed? Or a snowflake itself?
You can thank thank the 109 degree angle for that. At 109 degrees, there is no nice, easy geometrical form that can happen. Can’t very easily make a cube, or tetrahedron, or hexagonal structure.
And because these molecules are notoriously difficult to stack up, there is one more effect.
It takes more volume to “stack them” than just leave them liquid.
Ice floats!
Most informative, thanks!