edit with photoshop from lightroom - cannot re-open it with layers etc, all layers are merged

I do quite a lot of work with layered files in Lightroom, typically with a noise reduction layer or a sharpening layer – in both cases they are copies of the background layer.

I have to use Edit original (or Edit a copy) to preserve the layers, so any edits I have done in Lightroom will not be visible.

For noise reduction, I will apply a bit more than what I think is required, and adjust the layer opacity later.

Some Lightroom edits (like Clarity and Dehaze) will enhance noise, and if the image is too noisy when I've finished editing in Lightroom, I'll go back to Photoshop and increase the opacity of the noise reduction layer. 

But I can't see the result in Lightroom until I've saved the image, so some trial and error is required.

In short, Lightroom does not support layers of any kind or in any form. A Lightroom-processed file is always a flat file.

Photoshop embeds a flattened composite in all layered files (unless you disable that in PS prefs). This flattened composite is what Lightroom works with. All the layer data are just ignored - Lightroom can't read them.

The "edit original" option means Lightroom doesn't touch the file, it just passes it to Photoshop as-is.

Lightroom doesn’t support existing layers. So if you have a layered document in LR and you apply an LR Edit, then send it to Photoshop, this is a flattened copy of the original so that LR could apply it’s own edits. 

If you store the layered doc in LR and don’t apply an edits, you can of course open that layered document again in Photoshop and access those layers.