ProPhoto galleries are created as custom post types, much like regular posts and pages. Accordingly, they get their own top level WordPress menu item, right above the ProPhoto link.
Functionally what this means is that Galleries exist all on their own and have their own unique permalink that you can customize. In WordPress terminology they are a custom post type. If you go directly to a gallery’s unique permalink, you’ll see what looks like a page on your site with just the gallery. What makes it different than a post or page is that you can insert it directly into any other post, page, or widget area.
The workflow for galleries is to 1) create the gallery; 2) then insert the gallery. In “ProPhoto > Customizer > Layout” galleries are inserted as gallery widgets or selected as block backgrounds. When in a post or page, you insert a gallery shortcode from the ProPhoto dropdown.
Much like fonts, ProPhoto allows you to create “styles” for your galleries. The default styles all your galleries are set in “ProPhoto > Customizer > Galleries > Gallery Settings”. These are the styles that will apply to the gallery unless you choose an override style when inserting or creating a gallery.
Gallery size and resolution
ProPhoto will use the first image in your gallery to set the aspect ratio of your gallery view. This means that a tall first image will fix the gallery in a tall view, and a wide first image will fix the gallery in a wide view. You might choose to make a specific image first in a gallery, just to get the size/view you want.
Then, the gallery will grow/shrink to any resolution to fit. This means that on very large screens, all images will upscale to fill the gallery view. On small screens, all images will downsize to fit the gallery view.
Your image resolution will be optimized automatically for the best quality possible on the device. For very large displays, photos of as much as 3000px on the long edge could be used, but you’ll want to experiment to find an image resolution that looks good to you, without being too large.