Where can I find a complete QB Online user manual?

Quickbooks provides no such animal.

You can look on Amazon or at B&N  for a "QBO FOR DUMMIES", making sure you get the right version, keeping in mind the program is fluid and always evolving 

I do suggest a good start would be this 23 minute video from a respected contributor https://www.google.com/search?q=quickbooks+for+dummies&oq=quickbooks+for+dummies&aqs=chrome..69i57.1...

So frustrating.

We have a manual guide for QBO Global version dated December 2019. I noticed it is the first manual guide and no revision ever since. But we may still share it to our clients as reference. Most of the content are still relevant.