Roblox Studio is compatible with the Steam overlay, and you can bring this up in-game by pressing Tab and Shift. So, whether you're a creator or a player, you have your answer to whether Roblox is ...

Roblox has had in-game development tools via its Studio feature for a while, offering users the chance to create entire games that can then be published for other players and monetized.

Simple node.js program that can tell you (in small games, see limit section) the game instance of a player in a place and has been tested to find game instances on games with around 1k players.