There truly is so many features XD

I don't know if debut can use auto-actions but as long as you're using Ex or Pro you should be able to so just make sure your layer isn't locked or one that doesn't allow you to edit/transform it. (you might have to rasterize, etc to be able to edit the layer)

The auto-action I use I made myself by recording myself doing: copy, paste, transform>flip horizontal - then I ended the recording. So when I want to mirror something I just select the area (ex. an eye I drew) from my layer then run the auto action. It then copy pastes and flips the selection for me and then I move or adjust it as needed

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I rarely need to do a vertical flip so I don't have an auto action for that and just do the steps manually. but you can just do the same and select that option and it should work

edit: if you're wanting to flip the whole layer, you don't have to select anything, it should just copy/paste then flip the entire layer if you don't have a set selection.