Mixins are a powerful and important tool used in the Fabric ecosystem. Their primary use case is modifying existing code in the base game, whether it be through injecting custom logic, removing mechanics, or modifying values. Note that mixins must be written in Java, even if you use Kotlin or another language.

Since Fabric Loader 0.15, MixinExtras has become bundled in Fabric Loader, then you can directly use MixinExtras to better manupulate the mixins.

For a more accurate complete walkthrough on Mixin functionality, usage, and mechanics, view the Mixin Official Wiki. Additional documentation can be found in the Mixin Javadoc.

The Fabric Wiki also offers several articles that provide practical examples and explanations:

• Mixin registration
• Injects
• Accessors and Invokers
• Redirectors
  • Method redirectors
• Tips
• Examples
• Hotswapping Mixins
• Exporting Mixin Classes
• Access Wideners
• Reflection
• Interface Injection