Calendars in Outlook 2016

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Outlook version
Outlook 2016 64 bit
Email Account
I don't use Outlook, but my wife does and we are having a lot of trouble understanding calendars. A lot of web searching has not located the explanations I need, so here goes:
1. Can anyone point me to a good explanation (both simple and more advanced) of the way in which calendars, especially multiple calendars, work in Outlook 2016 -- including how to sync the Outlook calendar with an iPhone.
2. My wife has 4 or 5 calendars and she's not sure where they came from or what different purposes they serve. Some are empty and some appear to be duplicates (she does not share any calendars). They are My Calendars: Calendar - Outlook; iCloud: Calendar - iCloud, Calendar (2) and Home. Where can I find explanations of what the different calendars are, which one she should use and how to get rid of the others? And, in particular, what does the (2) mean after the second iCloud calendar?

Thanks in anticipation.
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