How to create a custom media file format on your own?

Hi, I taught myself software development by browsing the internet 8 years ago. From there, I know and have been able to create web apps and know a bit of best practices (design pattern, clean code, testing, etc). Used to be LAMP stack dev around 8 years ago, now I'm a fan of Serverless, JAMStack, and all javascript thing.

But, now I want to learn how to create a media file on my own. Like, how do people invent .EPUB file for a book? What's behind that .EPUB? As far as I know, EPUB is mostly html text only right (since I think EPUB is very fluid on all devices?) But how do people turn that html into a new media format called epub, and why? I'm very curious

Would love if you give me any links to book/learning references for this