pondering on legato, legatissimo, and non legato

I thought it's worthwhile giving it much thought about what is otherwise taken lightly but should not be.
Given the structure and principles of piano as an instrument, legato is something requires not only good control of fingers and wrists (and for that matter arms), but also good coordination and feedback with listeninng to the sounde created.
If you want good legato (bad legato means uncontrolled non legato or contaminated sound with overwrapped notes), you need to keep you wrists calm (not fixed but flexible though), keep your fingertips as close as possible to the keys, and keep keys pressed as long as just enough so that two consecutive notes sound connected to each other. This "just enough" is easier said than done. Never too short, never too long. It's a split second difference. The difference would be much smaller when it comes to legatissimo. The same thing holds for non legato. It is hard to describe it quantitatively. We need to learn by doing, i.e., playing and listening, and listening and playing.