So I have edited a page. Is the previous content lost?

Not at all. All pages have "history" which consists of a series of revisions. Each change (title, content, rename, file upload etc.) creates a new revision. By clicking the history button at the bottom of a page you can browse the list of all revisions of a page.

In principle the rule is no content is lost. This applies perfectly to pages - pages have history.

The rule does not apply to uploaded files due to limited file storage size. So Users (who have permission) can replace/delete files.