https://en.wikipedia.org/wiki/Vickers_Wellington
If you want to see a Wellington being constructed have a hunt around for the film referred to in the link. I have not checked but its almost certainly available on YouTube or the BFI site.
RE Shadow Factories: In general there were two types those that produced components i.e. hoses, tyres, instruments etc. and those that produced sub-assemblies i.e. wings, fuselarge sections etc. It was organised as a sort of pyramid with the components being made in literally hundreds of (often small) workshops. Sub-assemblies in larger works with final assembly in airfields such as (in the case of Wellingtons) Broughton Nr. Chester.