Here is a tool called Ragnarok that let's you edit values based on their name. You would open the reversed actionscript and could play with things based on the variable name:
Editing save files was big. The files were in a proprietary format called sol. All of the videos I see on that channel are just "download this save file with a completed game state", but there were tools for editing the save file yourself. Some games introduced obfuscation of data in the save file and the cat and mouse game began.
Oh, here's an example! Apparently the sol editor was a feature of Ragnarok, and maybe the disassembler too:
Here's a video of Niflheim, seems similar to Ragnarok. Maybe Ragnarok replaced it as a better tool?
Other than that it seems most of the cheats on that youtube channel are using cheat engine. While I definitely used that, I don't recall it being part of my go-to arsenal.