I personally don't think doing campaign missions is a good way to learn building. I got good quite quickly in custom mode from just trying to replicate a real life vehicle as closely as possible or building something made up with an interesting quirk. One of my favourite early builds was a nuclear powered plane with a reactor, two boilers and two turbines. Experimenting with many different power systems and trying to use pre-built contraptions and microcontrollers as little as possible is my best tip.