The Unreal Engine is used to create those vast virtual open worlds for games (and anything else). Version 5 especially is a thing of real beauty and detail (as the intro to the video shows).
It’s a lengthy tutorial, but does take you through all the beginner steps to get going and create your world. It’s a great tool to unleash unlimited creativity, whether for a game, or to share with friends, or to have a place to escape to that is better than the real world.
It’s completely free to use (until you exceed $1 million in gross revenue for a game you’ve published), and there is even multi-user editing, where you can connect multiple instances of Unreal Editor together to work collaboratively in a shared editing session, building a single virtual world together with your teammates and colleagues in real time.
See
Unreal Engine 5 Beginner Tutorial – UE5 Starter Course!
by Unreal Sensei on YouTube