By default the files are stored within the Library package, just like iPhoto.
If you're, as you say, a files and folders guy, then a couple of possibilities: use a referenced library - that is, you decide where the files are stored. I strongly urge against it as it's more work for exactly no gain and one huge loss - you can't use referenced files with the iCloud Photo Library.
The other possibility is to use some other app. I'm not being snarky here. Sometimes the app Apple installs, capable as it might be, is just not the right tool for your particular task. There are lots of Photo apps out there, each with different solutions to the basic problem. You may find one more to your taste.
The reason is that I run an automatic backup every night and I need to know which file/folder to specify so that my pictures will be backed up properly.
There are dozens and dozens of back up utilities. I'm not sure which one you use, but none of the significant ones will need to know which specific folder within the Library package needs to be backed up. They just compare with the previous back up and add anything new or changed. So, you might consider that as a possibility as well.