You've probably discovered this by now, but just incase someone else finds it helpful:
The answer to your first question is "kind of." You can view created sites under the Groups section in the admin center (the two people icon). User created groups are tagged as Office 365, for easier filtering. Groups are automatically created for a number of functions (i.e. for teams, plans, sharepoint, etc.). From the admin page you can add yourself as a member if you desire more information about a particular group. It does take awhile for this to take effect. . . .
I've been trying to get a handle on sharepoint online this week and so far have found these sites useful:
https://techcommunity.microsoft.com/t5/Microsoft-Teams/Can-Whole-Site-Admin-Access-All-Private-Team/td-p/64791 - about accessing private sites
https://en.share-gate.com/blog/office-365-groups-webinar - Old, but some really useful information about how groups work(ed). Might help answer your other questions . . .
