Alt + x,
Alt + y,
Alt + z
The Symmetrize tool extends blender's native Symmetrize with the ability to mirror custom normals.
Mirroring custom normals can be very tricky on the center edge loop.
MESHmachine's Symmetrize provides options to deal with this, but they are imperfect and often custom normals will need to be fixed manually in the center region.
No selection required, you only need to be in edit mode.