https://www.wired.com/story/this-prosthetic-limb-actually-attaches-to-the-wearers-nerves/

Most prosthetic arms use the person’s other body parts, like the shoulders or elbows, to power them, which limits dexterity. But the one the team demonstrated in the study was attached directly to a user’s own nervous system, allowing him to individually move each prosthetic finger at will.

The article is mostly about the newer tech, but at a couple points it goes into the basics of the older tech, if that’s what you’re curious about.