The answer to your question is tangible if mathematical input is of concern.

Entry-level plumbers may not do complex chores while skilled & experienced plumbers know that they can't line something without calculation.

Plumbing isn't for dum-dums...but you're right, shit does roll downhill.

I'd say there's more math in plumbing than in welding.

Welding's not math heavy. Welders weld.

Insight, as in reading what you gotta do before you even arc it isn't exactly math, but welding does involves thinking...yes.

Welding has its technicalities.Welding is a skilled trade.

Fabrication is fairly math-heavy. Fitter-fabricators do all the mental grunt.