If you watch the video "Building levels with snaps," you might find that you are unable to install progrids. in order to install it, in your project go to edit -> Project settings -> Package manager -> Enable pre-release packages. Then, go to window -> package manager -> click packages in project and change it to unity registry -> click the + in the top left -> click "Add package by name" and type in com.unity.progrids -> this should add it to unity but keep in mind that it is an experimental package with some bugs.