Buying a domain does not host a website. It only reserves a name. Hosting is where the files live. You connect them with DNS.

On SiteHost you add the domain to the site, then create the records we display (usually an A or CNAME) at the registrar. When those records verify, we issue HTTPS. Until then, use the preview URL.

Keep the domain at a registrar you like. There is no need to “transfer hosting and domain” as a bundle unless you want that convenience; it is not required.