The standard retainer covers uptime and security monitoring, dependency and platform updates, backups, support on your content system, small content and copy changes, and a monthly look at performance and search visibility.
It does not cover new pages, new features, redesigns or migrations. Those are scoped and quoted on their own. Keeping that line clear is what makes the retainer worth having: an unbounded monthly fee becomes a bad deal for you or a bad deal for us, and usually both at once.
Retainers are optional and run in monthly cycles. Plenty of clients take the build and run it themselves, which is rather the point of owning the content system.