What's the difference between photos and pictures on a 2015 macbook pro?

Everything on a computer is a file of some sort.

Generally, the way I see it, "pictures" are the files themselves handled by Finder.

"Photos" are those same files but handled in a different way, with different logic, by some sort of photo handling catalog, manager or some such.

I prefer "pictures" as I want total control of how the files are grouped, copied, classified, kept up with and so forth. With "Photos" you lack the total control Finder has over the files themselves, locations and such.

You can delete any user data you want; it's your machine.

I don't use a Photos app so I can't really answer your question. But, your quastion is one reason I deal only with files as supported by Finder. I'd advise you to start here: https://macpaw.com/how-to/photos-app-mac and/or ask here: https://macpaw.com/how-to/photos-app-mac