How do I find and use the filename on photos in Photos.app?
You can still see the original file name of a photo in the information panel, opened using cmd ⌘ I. It's right there under the photo's title.

It seems you cannot search for file names in the Photos.app search. What still works is creating a smart album (⌥ alt cmd ⌘  N) to search for a specific file name or a pattern. See for example this Macworld article.

The Photos app really hides the actual file names quite well. In the finder, you cannot even search for them using Spotlight unless you first open the package content of your Photos library and directly search in that Finder window.