DX lenses on Nikon Mirrorless camera z6ii

You get a "circular" vignette around a normal middle – the lens is made to illuminate an APS-C sensor, so it'll illuminate a circle around a ~16mm x 12mm (roughly) patch in the middle of your larger sensor.

No distortion, just less light or all the way black in the corners.

The Z cameras automatically crop to DX proportions, so you will see approximately the same image in the EVF as you would see on a DX DSLR. Assuming you have Vignette Control and Lens Distortion control turned On, you would also see a better view compared to through the viewfinder of a DSLR because it has been corrected. The only real difference is you have a smaller image with DX proportions - but through the EVF it would still fill the screen. On the Z6II that means an image around 10 megapixels. You would have the 1.5 crop factor.

With a DSLR, if you use a DX lens on a full frame camera, the viewfinder does not change so you see the darker corners that Guido described depending on your focal length. Wider focal lengths tend to vignette more - even a porthole effect, but many times as you zoom longer the vignette is reduced or eliminated. You don't have the option to view an uncropped image in the Z cameras - it is automatically cropped to DX proportions.

Whether you want to take that approach depends on what you are trying to accomplish. I'd consider it something you would use occasionally when you wanted a one lens solution and image quality was not critical. I don't think it is a substitute for a proper FX lens most of the time. The Z 24-70 f/4 is such a good lens I would not consider using the 18-200 instead of the 24-70 as my primary lens. If you want to use that lens, you might consider a Z50, but I'd still get the 16-50 lens that comes with the Z50 kit. It's a very tiny lens and better image quality than the 18-200. The 18-200 with FTZ is a bit bulky compared to alternatives.

The Z cameras automatically recognize it's a DX lens and put themselves into DX mode, and this is seamless. You do end up with a 10Mp image so you lose a lot of pixels. That means you cannot print as large as the same image shot in full frame mode.