> Each pixel? Why?

I use the Zarr format (for climate science data rather than microscope data), and I think this is just poor wording in the article. In the Zarr specification the metadata is stored separately from the actual chunks of compressed array data. So the metadata applies at the array level, not the pixel level.

> Wait - are there optical microscopes that can create 3D images?

I think so - they do it by scanning lots of images at different focal lengths to create a 3D section (I think?). There are whole projects just for visualizing the multi-TeraByte 3D image files produced - Napari is an open-source image viewer which opens OME-Zarr data.