How are muscles, tendons, and bones related?
They are the biological ‘layers’ converting structure into motion.
Turning the sequence around, the bones are the structural framework of the body, upon which everything else is built.
Bones by themselves are static, incapable of independent movement. The muscles provide the “engine”. They can move and produce force, but only in a limited range. The tendons are the intermediary connections between the muscles, which require flexibility, and the bones, which require rigidity.
The combination of the bone structure (essentially, many different levers) with the bidirectional motion and force of the muscles through the tendons allows skeletal organisms a much wider range of motion and dexterity than is available to non-skeletal organisms or static ones (corals).
A machine analogy could be an automobile,where the frame is the bones, the engine is the muscle, and the drive train is the tendons.