Standard assets package is a collection of assets, scripts, and example scenes can be used to kick start your Unity learning or be used as the basis for your own projects.
Unity used to have it in the game engine whereby you can import them by right clicking on the project tab and import. Unity has since removed it. But no fret! You can still download it in Unity asset store.
Download it in Unity's asset store here.
The package includes:
First Person Character Controller
Third Person Character Controller
Particle Example Scene
Sample 2D Platformer Scene
Cross Platform Input: Various mobile control rigs ready to drag and drop into your scenes.
AI Examples: Showing how - with the proper set-up - you can swap user input for a simple AI input