You place blocks that are already in a folder. Once you leave, the blocks you placed are stored in the server forever. An example would be this game: https://web.roblox.com/games/5889471811/your-last-thoughts-at-the-end-of-time The notes are stored on the server forever. Once a player leaves, the note is stored into the server.
Create a folder in workspace, and then when you use the tool that creates parts ect. the parts parents is the folder, if you mean that.