In version 1.25 we have introduced new StarterGame content. You can find the content in dev\Gems\StarterGame. We are also working on updated tutorial content. That particular file is not part of the new content. You can still learn a bit from the video, you will just have to use another map or generate a terrain.
You can use any grayscale tif file as a heightmap or auto generate terrain by selecting Generate Terrain from the tools menu in Terrain Editor. A properties window will open allowing you to set some inputs for the terrain generator. The terrain generator will use those properties and some random seeds to generate a natural looking heightmap that you can refine in the terrain editor and with the terrain tool.
Also note the Infinite Ocean component is not included by default when creating a new level. You can select the DefaultLevelSetup entity in the Entity Outliner and add the Infinite Ocean component to it in the Entity Inspector if you would like to see water, as shown in the video.