How Unity works with packages
Unity 打开项目时，Unity Package Manager 会读取项目清单文件 (1) 以确定要在项目中加载的包。然后会向每个包（在清单中显示为依赖项）的包注册表服务器 (3) 发送一个请求 (2)。包注册表将请求的信息和数据发送回 Package Manager (4)，然后由 Package Manager 将这些包安装到项目中 (5)。每个项目都有其自己的清单，清单中列出了要作为项目的“依赖项”加载的包。
Adding a package to a project requires an update to the project manifest, to ensure the Package Manager includes the package in the list of dependencies. Although you can modify the project manifest directly, it’s safer and easier to work with the Package Manager window, which manages the project manifest modifications for you.