How to mirror a rotated cube

I can mirror a cube but it looks not good when it is a rotated cube.
How to do that I now do shift D then S and Y -1

Blender 2.83.9

Just want to mirror with short keys for a better workflow.

I believe you could just use the mirror modifier, no?

Here is the manual page that describes its use:

Go into Edit Mode. Hit: A. Shift D. S. X X (or Y Y). 1. -. Enter.

Then maybe: A. Mesh > Clean up > Merge by distance.

Or just switch to Local Transform Orientation. Stay in Object Mode and follow your procedure.
Your shortcuts: , 6 (Shift D) S Y 1 - Enter

Nope does not work

X X is wrong does this not work

Had to change the pivot point then I can use the mirror modifier.

I have no idea how to mirror with short keys I thought normally it was Shift D S X -1 enter.

The reason it doesn’t work in your first post is because you have:
Transformation orientation: Global
Transform pivot point: 3D cursor (which is at the world origin)

You want:
Transformation orientation: Local

And then you have to move the 3D cursor to the center (at least on the Y axis) of your new cube. The easiest way: Tab into Edit mode, select one of the vertices on the inner open edge ring, Shift S - Cursor to selected.

Then select your half cube. Shift D to duplicate. S to scale. Y for the Y axis. 1 to preserve volume. And then the minus sign (-) to flip. Enter.

The Mirror Modifier uses the Object Origin not the Pivot Point (which could be the same depending on how the Pivot Point is set). Unless you use a Mirror Object, which then uses that Origin.

This copies on the y axis but did not mirror the object. The pivot point was set to cursor
And the local origin was set to the centered cursor.

Thanks for explaining .

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I know you’re confused why it is not mirroring. It should mirror. I don’t know why it is not. My best guess was that the negative sign wasn’t registering.

That should happen if you hit G to Grab, Y for Y axis, then 1 , and minus sign.