Skill is critical, parts and tools can be improvised.
While I agree with C.A.Y.: “… the combination of skills plus tools plus parts is what’s needed”, there are important exceptions. In some south asia villages, a highly skilled artificer [with a few assistants] can create a self-loading pistol, per day, without parts, and only the most primitive tools of drills, belt sanders and files. The steel is recycled from wrecked cars and trucks. The skill is what makes this possible. This town near the Khyber Pass makes one thousand guns per day. Look at minute marks 3:33 and 3:46 for the ammo and gun fabrication.
During WW2, Allied POWs [in German Stammlagers and Oflags] fabricated metal cutting lathes, shortwave radio receivers, photographic darkroom developing equipment and offset printing for counterfeit documents – all without the appropriate tools or parts – it was all improvised. The skill with working with the original equipment back home showed the way to the objective.
Conversely, in my fully-equipped machine shop, I have seen freshly graduated mechanical technologists and machine tool operators wreck instruments and equipment, ruin dies, moulds and tooling – and occasionally remove necessary appendages from their bodies. It was the skill [and common sense] that was lacking.
Skill is critical, parts and tools can be improvised. – Richard S.