Contractors use calculators or apps that handle that sort of a thing. The ones who are better at mental math simply do the mental math. No biggie. It only takes practice. Being a metric guy for 20 years, I had no trouble building some cabinetry, 20 years later, doing the whole thing in binary-base fractional inches. Anyone who's a computer geek should have no trouble there.
If you seriously think that most things are actually built down to a mm, you're crazy. Nobody in residential wood frame construction does it. Especially not when your materials are as dimensionally crazy as wood framing. Protip: studs aren't straight. If you're at the lumber yard, I wouldn't bet on 0.1% being within 1mm of the imaginary 3D box of ideal 2x4 straight lumber. Yeah, Europe doesn't have enough forest cover to build out of wood. Big deal, nobody builds down to 1mm using bricks and mortar either. I'd say that most US homes are built to the nearest 1/2 inch, if that. Heck, frame openings for windows are usually sized for the particular window being installed, not to any standard opening size. When it's time to replace the windows 30 years later, you end up swearing a lot :) I know that first hand.