A knife in a hand or a helmet on a head just works. An Auto Doc is surgery: more slots, more capacity, and a skill check. A failed surgery destroys the body and the other items in that slot. Complexity is compared to your surgery skill. Failures still train the skill.
Humans eat food. Machines eat mechanical parts. Some things, like Life Water or AI Dust, can addict you. Addiction ticks like food and is shown with your eating effects. It usually lasts longer than the food buff.