Have you seen these people with a weird tripod on construction sites ? They’re surveyor and have tools allowing them to do very precise measure (A guy told me that one of the coolest things he has done was to make sure railway track were aligned below 1mm on both side of 2 km bridge to build) I would expect that drawing a circle isn’t that complicated.

Then the rest is usual road construction, add temporary marking, close the road, do the measurement and build the roundabout