For ships, it’s all about angles. Look at some close up pics of an Arleigh Burke DDG. They’ve replaced round and flat structures with diamond and angled structures, even down to side rails, cleats, capstans, masts and gun turrets. this does not make ships invisible to radar. It reduces their RCS (radar cross section)so they might not be detected at longer distances and they might be mistaken for smaller vessels. The Burkes also have a system to reduce heat/infrared signature from exhaust stacks. Noise reductions under water...that’s touchy.
Fascinating; thank you.