yarn VS quilt-mappings

Yarn, still going with its cleanroom meant to protect against a mappings set that no longer exists.

Does Omori have something like Yarn?

I doubt that, especially considering Mojang started releasing obfuscation maps for Java Edition AFTER the introduction of datapacks. Furthermore, even before Mojang started releasing obfuscation maps there have been resources to deobfuscate the game, some dating back to the alpha days. In addition, many modders have opted to continue to use community deobfuscators to mod the game as a result of the terms of use attached to the Mojang mappings, and as such they are still under active development, yarn being the most popular (to my knowledge) community mapping project. None of these community mappings are in violation of copyright law to my knowledge, and I know for a fact yarn in particular isn't.

Download yarn https://github.com/FabricMC/yarn and OpenJDK 16

If you do wanna deobfuscate and decompile it tho, look into yarn or DecompilerMC

get FabricMC/yarn

Yup, they also still have: https://github.com/FabricMC/yarn, which is pretty nice but still rather restrictive as far as reversing more of the intersting code goes. This is understandable due to the nature of reversing a closed source software. Indeed I hope a group emerges that is strictly doing it for educational purposes or to find vulns and report them :).

Quilt Mappings. Quilt is a fork of Fabric made by many former-Fabric developers. Quilt Mappings are like Yarn, but without a cleanroom, and so may take inspiration from the Official Mappings. Its main purpose is to be like Yarn, but Mojmappier, or to be like Mojmap, but Yarnier - depending on who you ask.

Honestly the way I learned is just downloading the code (https://github.com/hube12/DecompilerMC or https://github.com/QuiltMC/quilt-mappings are probably the best ways right now) and poking around until I found what I was looking for.