robotic prosthesis already exist. exoskeletons exist, but are very limited and not really practically useful outside of very specific use cases. robotic arms do a substantial amount of work in manufacturing, just not attached to people. remember, a robotic arm might be able to lift a load, but that arm just passes the lobe on to your shoulder, back, and legs. you still need to balance the forces in your body. an infinitely strong arm in isolation would have very limited use without a full robotic skeleton to support it. so yes, these things exist, but no, not in the sci fi manner you’re thinking (nor would they be useful if they did exist). we have the technology, but physics in the real world is quite pesky.