Just Build Lol (JustBuild.lol) is a Fortnite simulator. It was released before Epic Games introduced the playground mode, so it was the most popular game to play in order to practice its Fortnite builds.
The Fortnite simulator lets you add keychains and change sensitivity, which you may have to do before you get started. Once you do, just click the play button and you will be able to practice your building skills.
In the simulator, players can place a standard wall, staircase, cone and floor. You can also set a trap, although I really don’t understand why anyone would use this option.