Prepare
Every page as its own file, every N pages as a file, or spreads cut down the middle into single pages. Spreads are detected from the page size.
Split a 40-page file into 40 files, or into chunks of 8.
Two-page spreads exported from layout apps become single pages, in reading order.
Output files are numbered so they stay in order.
The defaults come from your file. Change a setting and the proof updates before you let go of the mouse.