Roblox is a online open wolrd platform that is free to sign and join most expranses (wolrds / games) you can aslo make your own game by downloading the free studio exe this again is free to make your ownj wolrd / game you can code model and use pluginns to help you can do almost anything form data saving to banning players even banning a player for being part of commanity (used to be called groups pre 2020) such as a group of uses who may wish to harm your game and make othere players stop joining
after making an account and loading the software it is a simple a clicking crater and choosing form a black world or a template it will open an window with all the kit you need to get going you can add parts 3d shapes and change there data to move and resize colour and texter them as well adding scripts to edit how they behave etc some parts like using API,s your need to public the game before you can text in the studio or on the main player severs don't worry you can make it privet to stop anyone form joining it