I’m trying to make a robot so the arms need to be identical.
But I don’t want to copy and paste it and then have to rotate it, can I mirror a selection?
If the robot is supposed to be generally symmetrical I’d go with a Mirror Modifier while modeling, it works wonderfully.
But yes, you can also mirror selections manually in Edit mode.
Select and duplicate the arm (without moving it).
Make sure the 3d cursor is centered, and used as pivot point.
Press ctrl+M, then (probably) X to mirror across the X axis.