Sharp Edges in Blender

When displaying as smooth, the object’s edges and vertices are smoothed - excessively so. This is fine for bodies, and very much less fine for things like robots, or seams on coats, pants, etc.

The way you fix this is by marking edges as sharp. You can do this via edge angle (using the Edge Split modifier) or by manually marking edges as sharp (…also using the modifier)

Relevant Doc:

If you want to manually mark edges by angle, (say, to select them and manually remove those you don’t want to split) then you hover over the select menu while in Edit mode. The menu is on the bottom left, and there should be an option called ‘sharp edges’. When that is selected, all edges with angle > 30 degrees will automatically be selected, and you can adjust on the tools pane.

To mark edges as sharp, use Ctrl-E -> Mark sharp. This doesn’t actually do anything until you use the edgesplit modifier, and you really shouldn’t apply the edgesplit modifier until you’re done with modelling.