Ask HN: How are online code runner/playgrounds made?

For example, https://play.crystal-lang.org/#/cr, https://play.rust-lang.org/, or https://executeprogram.com

I'm thinking there are two options. Either having some kind of <Language> to JS interpreter that runs the code in the browser, or, running the code in some kind of container on a server and returning the response to the client. In which case, how are the servers secured against potentially dangerous input?