Maybe they just used a square (tool) or levels.

Geometrically the simpler and more precise way to make a perpendicular line is to mark two points an equal distance on the left and right, and then from them intersect circles of a larger radius above the line. Drop a perpendicular down to your starting point. That’s much more precise because you don’t need to have a radius 5/4 the size of another one.

But you can lay out rectangles on the ground with proper right angles with fewer workers if you use the Pythagorean theorem instead of trying to get a parallelogram out of ropes with the diagonals equal.