I know how it's done, I used to be a chef, at higher end restaurants than most people will ever eat at.
Yes, during service you're not busting out scales, and a lot of things aren't precisely measured. But anything where precision is necessary, you weigh it. Where it isn't, you don't.
Good luck ever finding a cup measure, or a measuring teaspoon in a professional kitchen though...
> In which case, it matters little if you use metric or not...
Well, you still will measure things, most cooks will do a stint in the pastry kitchen, plus in the higher end restaurants a lot of pastry techniques transfer over to the savoury side (ie. a lot of what we think of as "molecular gastronomy").
While metric vs imperial doesn't matter for accuracy (whereas weight is more accurate than volume if you're measuring anything that's not liquid, and even with liquids it's easier to measure volume incorrectly), metric is just easier.
On "the line" where food is cooked, measurements are mainly by hand and by eye.