I haven't found any docs or similar thread about the differences between Windows 10 Mail/Calendar app and Outlook for desktop.
However, based on my test, if I added a gmail account to Outlook even if I manually chosed the Google account type:
![enter image description here](https://cdn.statically.io/img/i.sstatic.net/ZJF1s.png)
Outlook will still add this account with IMAP (I think it's by design):
![enter image description here](https://cdn.statically.io/img/i.sstatic.net/jEapM.png)
As far as the windows 10 Mail/Calendar app, if we manually choose the Google account type, all calendars will show in the Calendar panel. However, there will be no calendars synchronized to Outlook if we add the gmail account via IMAP:
![enter image description here](https://cdn.statically.io/img/i.sstatic.net/GkeLH.png)
Therefore, if we use Outlook to add a gmail mailbox, Outlook will automatically use IMAP. As Roady said in the following thread, IMAP protocol itself doesn't support Calendars: Outlook 2016 - How do I add a Full-Function Calendar to IMAP Account?
It seems that you could only refer to the official docs to add the gmail calendars, maybe there is an add-in which can help you sync all the gmail calendars: Connect Gmail Calendar to Outlook