Syncing OneNote between iphone and ipad

Sign into OneNote on both devices using your Microsoft ID

This is how you locate the area to sign in - click on the 3 parallel horizontal lines

Then click on the Settings (Gear) icon

That will bring you to the account page - sign in with the same Microsoft ID on both

Then click on the 3 dotted lines below the battery icon on the phone - click Sync now on the pop up that appears