I don't get how this Variables thing works?

The GetService method will create services that aren't already loaded, so some services you can't get by name because they don't exist yet.

Most services are already created: BadgeService, MarketplaceService, LogService, and GuiService are a few. A few that aren't already created are the GroupService, PathfindingService, and DataStoreService; there are probably some others but I'm not bothering to check.

Some services also aren't named properly. ScriptContext has the name of "Script Context"; that space makes you have to use game["Script Context"] if you want to get it by name, and you may not remember that each time you want to access it.