Q. What are the things made of ?
It depends on what ‘thing’ it is of course but its usually a mixture of :
Metals (excellent to use as long as they’re not too heavy)
Plastics (excellent to use because they’re not too heavy),
Old parts (e.g. gauges or pipework)
New Parts (aged to look old)
Then you need a some glue, a bit of solder, maybe a few welds here and there and a lot of imagination. Top it all off with lashings of paintwork.