Ok I've just converted to a Mac from Windows and I am totally stumped by Apple iCloud and Apple Photos (Running Mojave 10.14.2) In Windows I had all my photos organised by date (eg filename 2018-12-20, y/m/d)then a description (ex: a photo of my 9 month old would be 2018-12-20 - Wallace 8 Months - Australia Zoo) but I can't figure out how to organise or change filenames on Mac. Changing the title doesn't change the file name and I NEED it to!! I want to back up to an external hard drive but everything recent will all be jumble when I do with not being able to change the file names. Help please!!!
Photos imported into Photos.app go into a large .photolibrary resource. You should consider it to be a black hole. You can't (and shouldn't) organize the the photos there in the Finder or rename them.
However, Photos.app will automatically show you photos by date (among other sorting styles). Also by geolocation (e.g. Australia Zoo), Faces (e.g. Wallace), and more. Separate metadata for each of these is much more organized and useful than one filename which happens to include all the data.
You state in the comments that you have a 'control-freak' nature. Well, fighting Photos is only going to make things worse. Either use Photos as it is, or don't use it, and organize your Photos manually, or by using some other app.