To download the (often referred to as "Pobierz Fabric API tutaj" in Polish), you should always use official repositories like Modrinth or CurseForge to ensure you are getting a safe and compatible file. Quick Download and Installation Guide
The Fabric API is a mandatory library for nearly all mods running on the Fabric Loader. It does not add content on its own but provides the necessary hooks for other mods to function. How To Download Fabric API In Minecraft 1.21.6! - Tutorial