Fabric

Fabric is a lightweight modloader. It has a separate API, required by most but not all Fabric mods in the mods folder. It has been worked on alongside the 1.14 snapshots of Minecraft Java Edition since before snapshot 18w49a (before December 5th 2018). It was created in 2016 as a hobby project of asiekierka.

It is currently supporting 1.14.4 and 1.15.2. It also supports the latest snapshot, 20w21a, & combat test (currently combat test 5).

Background
Fabric was created in response to the allegedly slow development of Minecraft Forge around Minecraft version 1.13. The developers consider Forge bloated, so the Forge API functionalites were either split out into Fabric API or discarded outright. Fabric itself is only a modloader.

Installing Fabric
Fabric Installation instructions can be found on the Fabric wiki here.

Note that this instruction does not install the Fabric API, which is just a mod on curseforge to be put in the mods folder.

Using MultiMC
MultiMC was the first launcher (albeit third-party) to support the Fabric Modloader. There is two known ways to install Fabric for MultiMC. The "Fabric Install Button" and "Importing From Zip" methods. The button was added in August 11 2019, version 0.6.7.

The Fabric Install Button method is as simple as creating a new 1.14+ instance on MultiMC, then editing the instance to install Fabric how you would Forge or Liteloader for 1.12.2 and prior. By installing Fabric you are installing the Fabric Loader and Intermediary Mappings. Once this is installed just put your mods (including Fabric API) and it should load up correctly stating Fabric after the Minecraft version such as "Minecraft 1.14.4/Fabric."

The Importing from Zip method. requires you to go to Fabric's website here: https://fabricmc.net/use/ and selecting the correct loader version and Game/Mappings version, once you have selected the versions from the dropdown box you have two buttons to choose from, Download MultiMC Instance or Copy MultiMC Instance URL. The Download instance button will give you a zip file you can download to your local hard drive and import with MultiMC, while the Copy URL button copies the URL to your clipboard. Both need you to put a link of either the URL or the zip file folder location to set it up this way. Once the instance is created, just add the mods (including Fabric API) and it should be good to go.

MultiMC does not have a "Forge Install" button for 1.14+ because of the changes to Minecraft Forge in these versions, so as of now Fabric is the only easy way to get mods working in it.

Vanilla launcher
The Fabric Loader can be downloaded from the official website. Run the JAR file and follow the instructions. It has an option to create a new profile, which the user can select and run.

Installing mods
Like Rift, or Forge for Minecraft 1.13.2, Fabric does not read mods in individual folders. While Forge might have been able to read folders from 1.12.2 when they were put into it, Fabric cannot.

Modpacks are supported in the same format as Forge modpacks. The only compatibility issue would be the mods themselves.

Optifine (OptiFabric)
Once a modded Fabric setup has been made, download Optifine and put the installer jar into the mods folder. After that, download OptiFabric and put it into the mods folder too. OptiFabric helps Fabric load optifine correctly.

The installer file can be used as is or extracted, but you can use it extracted too. In either case, OptiFabric is required.

Mod Menu
If you're new to Rift or Fabric you'll notice there is no 'Mod Options' button like with Forge. This is due to the option not being built into the modloaders/Fabric API, it is instead a separate mod, this is due to keeping things lightweight as well as for specific limitations if it were bundled. Also players could consider it optional (similar to Fabric API where it is used for some mods but not all in terms of how optional it is) if they aren't interested in knowing how many (or what in particular) mods are installed, or tweaking configs (if the mod developer has configs and has made it accessible through Mod Menu). You can download Mod Menu to take advantage of these benefits here.

Twitch support
More Modloader support for Twitch has been wanted by players. Before this support becomes official, these two methods are used:


 * 1) The first unofficial method (dubbed the 'Unofficial Vanilla to Twitch Method') allows users to unofficially get Fabric to work (but you can't export to modpacks, and the profile doesn't stay so features are limited). This was the only support available at the time besides the waiting of Twitch launcher Development teams to implement Fabric into the launcher or Mod Developers to find a way (or without the need to manipulate more than was necessary) to make this possible with the launcher (this method is now outdated, has less benefits and isn't very stable).
 * 2) The second unofficial method gives more functionality and more natural support using a mod called Jumploader. It works from 1.14 to even 1.16 snapshots.

Both methods will be listed here for those interested. There is also tagging for Rift & Fabric mods on Curseforge.

Vanilla to Twitch (found out by SuntannedDuck2)
The unofficial method is possible if you have a Vanilla/Minecraft Java Edition launcher with Fabric already installed. If you have the .fabric, .mixin.out and Fabric Loader available in your .minecraft folder then you are ready to use them for this method, the method won't work without these. Now that you have these files/folders, you want to copy the .fabric and .mixin.out folders to the 'Install' folder (on in the folder for that instance you want Fabric to be setup for as it will likely read those files and the mods from there) of your Twitch launcher files. This is to provide the necessary aspects of Fabric available to the game/launcher. Once you have copied them you'll want the Fabric Loader version (snapshot or release both work) that is in your Vanilla instance and to copy it and place it into the 'Versions' folder. This is the profile/version the launcher will use to load the game similar to a Forge or Vanilla setup instance. Now just load up the launcher and you'll see the profile there ready to go, just make sure to have Fabric API and other necessary mods and you ready to play.

Warning: This method is known for the profile not always sticking to the one you selected and reverting back to the previous. Modpacks can also not be exported using this method.

Notice: The Fabric Loader may be the only thing required but it is useful to have the .fabric and .mixin.out folders/files so the game doesn't have to load these for the first time meaning it just has to read the files and continue on.

This method is useful for snapshots when using the Fabric Loader (such as the 1.16 snapshots) but otherwise it is recommended to use Jumploader for better results and release versions from 1.14 to 1.15.

Jumploader (by Comp500)
Jumploader is a method that is similar to installing a mod in a Fabric environment and allows for natural access to features of the launcher like using Fabric in the Twitch launcher, exporting modpacks, the profile staying still instead of reverting back as examples.

To setup Fabric in Twitch with Jumploader the user will want to download the mod first which can be done so from here: https://www.curseforge.com/minecraft/mc-mods/jumploader, once the user has the mod, they can create a new instance in Twitch or use an existing but it is more recommended to use a newly created for better results due to how Jumploader works. When creating a new instance in Twitch it will be important to consider the game version (1.14 to 1.15, 1.16 can be tricky to setup when using Forge and selecting profiles for new instances) and the Forge Modloader dropdown box. Jumploader considers Forge and allows Fabric mods to work, it does not allow Forge and Fabric mods to work alongside each other. So if you have Forge mods in an existing instance they will likely be ignored. Using Jumploader without a Forge Modloader version selected might work but it is best to try this having not considered the above "Unofficial Method's" Fabric Loader consideration.

Once the user has Jumploader (and Fabric API if needed) in the mods folder, a Forge Modloader version selected and other Fabric mods (works for release versions, the same cannot be said for snapshots as Forge isn't available for those, if you want snapshots to work it is best to just have a Fabric Loader version for snapshots and ignore the Forge Modloader versions when the instance was created) the game should load up successfully with the Fabric mods enabled.