Can it work with an existing website?

Sometimes, and it depends on what you are running.

The data intelligence layer can often be connected to a site as it stands, because Search Console and server logs do not care what built the site. The AI operating layer needs a codebase it can actually work in, which is where an existing site either fits or does not.

That assessment is one of the first things a walkthrough covers. It is a real question with a real answer, not a formality.