At the moment there are nine different OneNote versions: OneNote 2016 (Office version), OneNote 2016 (Free version), OneNote Online (browser version), OneNote Windows 10 (UWP) app, OneNote for MacOS, OneNote for iPad, OneNote for iPhone (different features than iPad version), OneNote for Android and OneNote for Windows 10 mobile (not maintained any more and probably discontinued after end of support for W10 mobile).
Although Microsoft is tending to declare all OneNote versions part of MS Office, actually most of those are separate and free apps. You may download OneNote without owning a MS Office license or subscription from the Mac App Store, Google Play store or iOS App store for free. The UWP app is part of every Windows 10, independent of MS Office. There is even a slightly stripped down “Office” version of OneNote 2016 offered as a free download on www.onenote.com.
But recently Microsoft is adding extra features to most OneNote versions that only get activated in conjunction with an MS Office 365 subscription. So there are two slightly different versions of the Windows 10 OneNote app for example.