How do I open .ONE and .ONEPKG files?

You absolutely need a Windows desktop version of OneNote, ideally OneNote 2010, 2013 or 2016 that comes with MS Office. The free version of OneNote 2016 and the Windows 10 app at least offer a limited handling of .ONE (not .ONEPKG) files. All other versions like OneNote for Mac, iOS or Andoid can’t handle OneNote files at all. They absolutely need notebooks to be stored in the cloud in a special, hidden format.