Where is data on a website stored?

when you sign up for a website where is the data stored

in a database most likely

Codecademy runs a webserver (could be nginx), which has an endpoint to which you send your register request, which is then validated (unique username, password long enough, stuff like that), if the data is valid, the data is stored in the database and a response is send to your computer/browser.

So, that would be an example of server-side verification, and not client-side authentication, correct?

you start with a reference word (that), i can’t deduce what that is referencing. JS runs in the client browser, so JS validation is client side validation.