Kids make maps. Stuff in the maps is sold for Roblox bucks. Roblox bucks cost money to buy. The kid who makes the map gets the Roblox bucks, and can sell them. The problem is you only get 30% back when you sell a Roblox buck.
So kids spend time making big maps and servers, buying ads, getting shoutouts on YouTube/whatever, and Roblox takes a 70% cut from all of it