Photos library: change to folders by date rather than file name

It looks like you were running Photos 5.0 in Catalina, and no longer Photos 4 on Mojave. Catalina is renaming all image files to filenames made of 32 hexadecimal digits, and the folders are the first letter of the hexadecimal code. This is not meant to be read by humans to access the photos.

TD suggested to export the photos, using File > Export. One of the export options is "Subfolder Format: Moment Name". This will group the exported photos according to the location and the date into subfolders.

when I go the library in Finder -> Pictures -> Show package contents -> originals

Here's a tip: don't go there. Accessing your library in this way is simply not supported. It's a bit like lifting the bonnet of your car. It might not look right to you, but it's designed to work that way, and if you re-arrange it then the car won't go :). So basically how the back end works is of no concern to you, as you never ever access the back end.

Basically, if you choose to use Photos you are opting to never access your images via the Finder, but to do so via the application and it's various sharing mechanisms - which cover pretty much every eventuality.

If you do want to leave or migrate away from Photos you would then export using the options under the File -> Export command.

If this date arrangement is important to you then the Photos app is not for you. There are plenty of 3rd party options, so feel free to check them out.