Hello Izuruu, I recommend you to consult tutoriels like this -->
Parallax Mapping in RPG Maker MZ
to fully understand how it works.
You're going to need a plugin as well if you want an open world map with a scrolling camera like the Galv Overlay plugin you can find here -->
Apart from that you will just need your natural skills on Photoshop, Photofiltre, Gimp or your prefered software !