Depends on what level you’re at. I had a friend help us install a new septic system. Holy shit was there a lot more to it than I ever guessed. Had no idea surveying gear was required.

At a higher level, same guy drafts plans for military bases and hospitals. Then he goes in the field to troubleshoot issues. Plumbing includes gas if applicable!

Most of us are familiar with, if not doing the work, the principles behind fixing a leak, replacing a sink or toilet, etc. I can do all of the above, but I don’t know the fine points in doing it right. There’s far more to residential plumbing than most see.