How can I imagine the Ktor websockets? Are they so that the browser permanently shows "refresh" or are they like a JavaScript file so that the user doesn't really notice them?

WebSockets can be used for a variety of purposes, they just open a persistent connection between two parties which you send data on in both ways (in contrast to the usual request/response model). They don't result in a "permanent refresh" in the web browser, if used on a web site displayed by a web browser.

OK thanks but I have a problem using it. When I connect the server throws an classnotfound error. I added the maven dependency but the jar was empty