My "Collection" does have its own Battle System, but during the demo, I spend some time to explain a lot of stuff from the Default Battle System. The custom Battle System I use extends the Default stuff heavily, but still relies on it for the basics. Its a pretty easy way to see what else you can do with just the Default stuff just so you can build on your understanding of how it works. This is especially true with Event Pages in the Battle System, which can be pretty confusing at first. Other battle systems may or may not have anything to do with the Event Pages, but that depends on each of those battle systems.
The "Collection" also explains a ton of other things about the Default Editor that a lot of people dont know about, so it is a good place to start if you are also looking to learn how to push RMXP to its limits by using just the basics of everything it can and can not do.