In construction you are using raw materials to build something to spec. You cut, trim, nail, mud, sand, paint, etc. in order to take a pile of sticks and BUILD a house. (most, if not all could be considered skilled trades)
For you Lego's, did you take a bunch of generic Lego's and build an airplane, or did you buy a Lego Airplane kit and follow the instructions? One of those I would consider assembly and the other I would call building.
There is a fine line between building and assembling, but I would usually give someone credit for building something if it took a good bit of ingenuity to accomplish.
This is one of those fine line I mentioned above. You took raw materials and built/i] a car. Then again, it's called an assembly plant because that's where all the finished components are brought together. But a lot of the jobs could be considered skilled trades and there is always some re-work that has to be done since not all parts fit together the same way. Then again, each person has a small job, which contributes to the final product; if you installed the dashboard did you build the car? That's a toughie, but I give the hat tip to skilled trades.