Every page in a notebook is a real page size, so what you export is exactly what you wrote on.
Paper
Five rulings: plain, lined, grid, dotted and graph. Lined paper has a margin rule down the left the way a printed pad does. Graph paper is a fine grid with a heavier line every fifth cell. Change the paper from the notebook's menu and every page in it follows.
Page size and orientation
A4 or Letter, portrait or landscape, both from the notebook's menu. Changing either moves every page to the new shape and the ink is laid back onto it, so a page written in portrait is still a page in landscape.
A notebook that was built from a PDF keeps the document's own page size instead, and the size and orientation items leave the menu, because the pages have to match the document under them.
Defaults for new notebooks
Settings, Notes, then the Handwriting section: the paper, page size and orientation chosen there are what a new notebook opens with. These defaults sync, so a notebook made on the phone and one made on the desktop start on the same paper. The choice inside a notebook is its own and does not change the defaults.
Dark mode
The sheet turns dark with the app and the automatic ink turns white, so a page written at night reads in the morning. Ink in a colour you picked keeps its colour.