The site's pages are created and published here.
Editing is not publishing
Every page has two versions: the draft you edit and the one that is live. Saving changes only the draft.
So you can edit a published page safely — visitors keep seeing the previous version until you publish. And if you saved a change and cannot see it on the site, it usually means it has not been published yet.
A page address is unique within a site
Two pages on one site cannot share an address, but two different sites can each have their own about page.
If an address is reported as taken and you cannot see a page using it, you are probably on a different site — or that page is archived.
Archiving is not deleting
An archived page comes off the site but continues to exist, with its content and its address. Which means its address is still taken.
Earlier versions are kept
Every save creates a previous version you can return to. Separately, in the Studio you can make a named checkpoint — "before the redesign" — which stays until you delete it yourself.