The problem lies in Fusion 360.
If you export using file export as .3mf then the refinement (poly count) is set to low. Thus it creates a low-quality circle.
If you export through the mesh tab, select 32d print and make sure the refinement is high then it exports a nice-looking perfect circle.
STL is unit-less, but everything that cares about units will pick one.
Typically, the unit picked is mm.
But even if that's not the case, you can always scale it with a unit conversion ratio after importing it.