...a folder or sub-folder is represented by a "manilla-folder" thumbnail with badges showing the quantity of contents.

Click/touch any folder image to proceed further into subfolders.

Orange counters indicate the # of sub-folders; green counters indicate the # of images; both, together, indicate the # of sub-folders and the # of images contained, therein. A thumbnail of every image in the current subfolder is automatically created if it does not already exist, each linking to a larger version in a touch-enabled fancybox 'lightbox'

Once you click/touch an image, you can then navigate through the sub-gallery in any of three ways:

Close the image-preview with the [esc] key, or with the × in the top-right corner

Navigation context (e.g., "breadcrumb trail", "link-back-to", etc.) is interactive at the top of the page

Image processing in Affinity Photo, batch processing in XnConvert, and graphic design in Affinity Designer

Primary source derived (and extensively modified using BBEdit on a Mac Pro) from Harry Ghazanian's most-excellent folioGallery

Unfortunately, Harry G has suspended development of FolioPages. I originally used v1.3

Until I get my step-by-step on my blog, see: github.com/stvnhg/foliogallery for more details, and example code

Thanks, Harry