As you build your game, Unity stores a lot of metadata about your Assets, such as import settings and links to other Assets, among other information. If you want to transfer your Assets into a different project and preserve all this information, you can export your Assets as a Custom Package.
See PackagesPackages are collections of Assets to be shared and re-used in Unity. There are two types of packages supported in Unity: Asset packages and packages available through the Unity Package Manager (UPM). Packages available through the Unity Package Manager are a fundamental method of delivering Unity functionality. More info
See in Glossary for general information on using packages in the Unity Package Manager (UPM) and Custom packages for information on creating, maintaining and deploying custom UPM packagesA Package managed by the Unity Package Manager.
See in Glossary.