Mainly through hacks like Brutal Mario (come on, everyone was inspired by it), the VIP series,
, A Strange Mission, etc. and by various people like Ersanio, Roy, Yoshicookiezeus, Sonikku, etc. which inspired me to do SNES ASM (and programming in general).
I don't remember when I learned ASM, though, but I think it was with Ersanio's ASM tutorial where I made my first advancements + tutorials such as Iceguy's tutorial.
The second advancement was to read the SNES Developer Manual but I don't know when I read that. It must be either by learning how CC-DMA works (where the SA-1 readme redirected to) or by the second ASM Workshop (which contributed to my ASM skills a bit as well). That made me learn about stuff like about Mode 2 which made me realise how certain games managed certain effects (everyone talks about Yoshi's Island but the train stage of Tiny Toon Adventures makes use of Mode 2 as well
Starting a couple years ago (when I started to used BSNES+ as my primary debugger), I was interested in how games handle certain effects which is why I investigated them with a debugger (and still do). The result: I learned how Yoshi's Island handles its colour gradient and parallax HDMA (which brought birth to "Scrollable" HDMA Gradients
and Parallax HDMA Toolkit
), I learned how SMB3 (SMAS) handles transparent water graphics (colour half-addition on layer 3 and sprites and layer 1 are both on mainscreen and subscreen) and how Super Metroid handles dark rooms (though I have speculated that it used colour subtraction especially since I noticed those rooms use two foreground layers years ago).
Okay, my layout looks ugly.