Well although I am now retired, looking back over many years of work in the Plumbing/Central heating/Gas fitting industry, what I can say, is that I had bags of opportunity to gain experience in nearly every field of Plumbing, except ships.
Being frank the companies wanted workers, its as simple as that.
In the big companies depending on what job you had, you could perhaps get away with having less knowledge than you could in a small company. In the bigger companies you sometimes had some one to ask how to do things, in the smaller it was usually all down to you.
As I said though, a lot depended on the job you did for the company, if you where supervisory it could be quite a difficult job, as usually, you where responsible not only for production and manpower control but also the technical supervision of the work, liaising with the various managements to try and solve all kinds of problems.
They looked upon you as being the expert company so relied on you to tell them of anything wrong on the drawings and perhaps suggest ways to improve or do things a different way.
Some site work was usually just a case of following drawings. Not always of course but many times.
The problem was, you where usually required to work much faster, house bashing was like working on a factory production line. You could go in first thing and not stop all day except for perhaps 1/2 hr lunch and drink of tea while working. You where also usually responsible for the quality of your own work. And it got inspected many times by different people. To many complaints and out you would go.