Basically, yes. Tendons and muscles. The bones are just the structures underlying it all. They’re like pieces of wood coming together at common points. The muscles, tendons, and ligaments then act like giant rubber bands pulling this direction and that, stretching across those bones, and often opposing each other between resting and flexing states.
Also, as you mention... it all really is pretty flimsy. This is why so very many people experience shoulder and knee trouble as they age, and hips too... or why were so easy broken after accidents and impacts.