not to do with units.
As mentioned by the other poster, AutoCAD is by default
unitless in model space (unless you’re working with fractional
inches/feet-inches). One unit could be a meter, could be a
millimeter, could be a kilometer. Draw your objects (2D or 3D)
full size based on what units are appropriate i.e. don’t draw
bolts with 1.0 being 1 kilometer.
Paper space is always millimeters for metric drawings and
inches for imperial drawings.