You can select a part of a track in a track's editor. Then you can drag it to another track's editor and drop it there. The part will be copied.
In many cases you'll want to keep the part at the same horizontal position, for example when you're editing the best parts of multiple takes together into a single track. This can be done easily by keeping the Shift key pressed while dragging the part.
Note that the Shift key must be pressed before you start moving the mouse.