How to provide clients with an admin portal on a custom from-scratch site?

I am in the process of becoming a self-taught web developer and working to understand the various paradigms of creating and deploying websites. As I currently understand it, there are three (VASTLY OVERSIMPLIFIED) paradigms:

• Designed and managed through a CMS platform (simple/limited: Squarespace, Wix; complex/powerful: Wordpress, Joomla)
• Static site created from scratch (custom HTML/CSS/Javascript or purchased/modified template)
• Dynamic web application (including server-side logic, databases, likely front- and back-end frameworks, etc)

The way I see it, the first and last ones include easy ways to give some administrative powers to your clients, so they can add new content (as one example). CMS already includes a full admin portal, and web app frameworks like Django typically allow you to provide an administrative interface for loading new content.

How would you handle this with a custom static site? If you're typically just loading static HTML/CSS/JS files onto the client's web hosting, it seems like they would need to loop you or another developer in every time they wanted to make a change to the site. Is this true? Or is there a way of giving them some sort of interface to make changes?

Also if any of my assumptions above are wrong, I will gladly take correction!