how to organize photos inside an album?

If you right-click an album title in the sidebar you get a Sort option with 3 choices. One of these is an alphabetical sort by title, but this only works if you have added a Title to each picture. It doesn't seem to do anything logical when sorting an album with no Title on the pictures.

Indeed, in most recent versions of Photos the "Sort by Title" will only sort the photos that have a title. It has been much better in Photos 4 on macOS 10.14 Mojave. If we still have Mojave installed, Photos will use the filenames as default titles, and we can sort the photos by the filenames, if a photo has no custom title. On Mojave the "sort by title" is behaving like in Aperture or iPhoto.

See: Sorting Albums by Title in Photos - Changes in Photos 5.0 on Catalina