Two files, two completely different jobs
The most common mistake in indie publishing is treating these as one task.
The print interior
Locked to a trim size, with mirrored margins and a gutter that grows as the page count does, so words don't disappear into the spine. Running heads, correct front and back matter order, chapters opening on the right hand page. Then embedded fonts and the right colour space so the printer renders what you designed.
The ebook
Reflowable, because the reader picks the type size. That means real styles instead of manual spacing, a linked table of contents that works, and a layout that survives a phone, a tablet and a six inch e-reader. Anything you positioned by eye will move.