DaemonUmbra

You have two choices, either you put it somewhere and then each of your mods points to where you put it, or you create a multi-build project

It won't show as a folder in the file explorer, it is a Gradle task folder

1.12 is no longer supported on this forum. Please update to a modern version of Minecraft to receive support.

13 separate instances of [net.minecraft.item.crafting.RecipeManager/]: Skipping loading recipe ... as it's serializer returned null in that debug.log, and many more failing because of missing ingredients/tags

1.12 is no longer supported on this forum. Please update to a modern version of Minecraft to receive support.

You misunderstand world.isRemote (you have it backwards)

1.12.2 and not even Forge.

1.12 is no longer supported on this forum. Please update to a modern version of Minecraft to receive support.

Version-named folders aren't a thing anymore, use Installations instead

The jar not being there is intentional. Hmm... I’m wondering if having non-ASCII characters in your username is causing issues

Delete this file

What happens if you uninstall all Java and only install 64-Bit Java 8?

Close the launcher, does the installer work double clicking now?

1.8 is no longer supported on this forum. Please update to a modern version of Minecraft to receive support.

OK.. let's try the old classic... have you tried restarting your PC?

What does (Get-Item -Path Registry::HKEY_CLASSES_ROOT\.jar).GetValue("") report in Powershell?

Was the launcher closed when you ran the installer?

Show the list of Minecraft versions

Please provide a screenshot of your launcher

That looks right... have you closed all your Explorer windows after running JarFix?

You're not supposed to have a jar file in there, just restart your launcher and Forge should be in the list of Minecraft versions.

Ok, do me a favor, open powershell and run this command: (Get-Item -Path Registry::HKEY_CLASSES_ROOT\jarfile\shell\open\command).GetValue("")

Yes, but is it still broken for double clicking the jar?

Is it still broken when double clicking?

Optifine for 1.13 and later isn't a Forge mod, it's a ModLauncher (the thing that Forge runs on top of nowadays) plugin. It won't show in Forge's mod list.