4-ch https://4-ch.net/img/res/3026.html said it : F=ma of course, so based on this, how does the body then formulate locomotion cycles that moves regularly in cyclical manner, and what are the force involved and how are the directions acquired?

Looking specifically how Opensim here produce that gait or whatever existing mechanism exist.

It should be pretty straight forward, and could even simplified into maybe 4-5 parts just for me: body, head,neck, upper and lower legs and arms, ribcGe,shoulder spine and maybe abdomen

simple basic equation should do