What does webcontainers exactly mean? Browsers don't allow me to call arbitrary other urls since the called server needs to allow with cross domain request head

What does webcontainers exactly mean? Browsers don't allow me to call arbitrary other urls since the called server needs to allow with cross domain request headers. What happens if I close browser tab? does that make the webcontainer go poof? What about memory and CPU requirements? The idea sounds like a great hack, but the whole point of cloud IDEs is that it uses someone else's resources. I could get a 64 CPU with a bunch of H100s to dev on, run some script in background, and shut it when I'm done.