An eBook is not a print book in a different file format. It is a fundamentally different object, and treating it as an export is why so many independent eBooks read badly.
A print page is fixed. An eBook reflows: the reader chooses the font, the size, the line spacing, the margins, the background colour, and the device might be a six-inch e-reader or a phone in the dark. A properly built EPUB accommodates all of that. It uses semantic structure so headings behave as headings, relative sizing so nothing breaks when text is scaled up, and images that scale sensibly rather than overflowing.
We build a navigable table of contents that works in the reader's own navigation menu, not just as a page of links. Internal cross-references, footnotes and endnotes are linked so a tap takes the reader there and back. Metadata is embedded correctly so the file identifies itself properly in a library of hundreds.
Then testing. Every file is checked on multiple devices and apps, at several font sizes, in both light and dark modes, and validated against retailer requirements before submission. This is what prevents the rejection emails and the one-star reviews that mention formatting rather than content.
For books where the design must hold picture books, cookbooks, anything heavily illustrated we build a fixed-layout edition instead, which is a different technique and quoted separately.