A bit of both, I'd say.
CMS won't teach you the practical details of the trade but it will provide a broad overview of various systems (braking, transmission, suspension, etc.), their locations, and how they interconnect.
The puzzling element comes from troubleshooting the vehicles problems. At first you're told exactly what to do for each work order, but as you progress the customer descriptions become more vague sometimes, forcing you to test-drive the cars and inspect the condition of individual parts.
The game is all sorts of awesome, offering a unique premise, satisfying game-play loop, and surprisingly sophisticated presentation values. There's a demo on the Steam storefront if you're curious to find out more.
It's one of the top-tier Simulation titles on the Steam marketplace, imo.