Mirroring is part of the Transform operations. The action mirrors any face, body or group to a mirror plane.
The Mirror flow
1. Tap on Transform/Mirror
2. Select the faces, bodies, and groups you want to mirror
3. Tap on Next
4. On the Preferences Panel set if you don't want to have a mirrored copy of the selected items.
5. Select a planar face to be the mirror plane.
6. Tap on Done