I've still never heard a satisfactory explanation for how in the hell two parts of a bone, broken such that they aren't even touching, can find their way back to each other and heal. My son broke his collar bone, and the hospital sent him home in a sling. When I looked at the x-ray, I couldn't believe that's the correct treatment. But a month or two later and he was good as new. Absolutely blows my mind every time I think of it.
I broke my collarbone about 30 years ago. Then had it partially rebroken when a friend forgot about my healing bone and greeting me with a shoulder grab from behind. (He felt like shit afterwards). I got the most basic healthcare a free college clinic could offer. Today, you can only tell I ever broke it when I'm shirtless and at my skinniest weight.
Mine ended up healing with a 15mm overlap. So it is a weird shape. It gives me 0 issues however.
They sense other bone tissue and grow together I guess. Same for severed nerves, up to a point.
It starts with bleeding. The blood in the region of the bones forms a squishy mass that is not bone, but through which cells can flow. Specialized cells deposit calcium into this mass, guided by hormonal and chemical signals. If you imagine the two open bone ends as emitting a sphere of chemical signals, where the two spheres interfere constructively is where the 'signal' is strongest and where you want to deposit calcium. Bones are not just calcium of course, but that's the gist.