FBX is a universal 3D file format for sharing model data (geometry, textures, animations) across tools, while a Unity 3D model is optimized for the Unity engine with engine-specific settings like colliders or scripts.
FBX works with Blender, Maya, and other software—great for cross-tool collaboration. A Unity model, however, includes additions (e.g., scripts) that only Unity recognizes, making it functional for game development in that engine.
To use a model in Unity: Export it as FBX first (Unity imports FBX seamlessly), then add Unity-specific components (like colliders) to turn it into a complete Unity 3D model.