Hi. I am fairly new to hacking, but I already know how to edit levels, tilesets, backgrounds, and some other stuff. However, one thing that I'm curious about is how to edit the physics of Mario/Luigi.

I know that there's a Mini Mario physics box in every area, but I want to do more than that. Say I want to change the walking speed. Where do I go and how do I do it?

You need to learn how to create ASM Hacks https://nsmbhd.net/thread/1281-how-asm-hacks-are-setup-tutorial/