[José Manuel Hermo Barreiro] has spent many many hours crafting these tiny engines from hand. Every single piece is custom made specifically for the engine it is going onto. He has created aircraft engines, car engines, and marine engines that all actually run and are the smallest of their kind in the world.
At one point in this video he stands in a room with several engines lined up, all running smoothly and considers that there are possibly over 15,000 hours of work right there in front of him.
Here’s a video specifically about the 12 cylinder construction.