The Package Manager is the official package management system for Unity, and allows users to install optional features, assets, and tools, in to a specific Unity Project.
The Package Manager can be opened by selecting Window > Package Manager from the Unity menu.
You can use the package manager to:
- Install optional official packages, such as the new Input System, the Post Processing package, and the Unity Recorder.
- Download and install third-party assets that you have purchased from the Unity Asset Store.
- Update or uninstall existing packages in the project.
For more information on the Package Manager, and how it works, see the official documentation (https://docs.unity3d.com/Manual/upm-ui.html)