Site is not listed in site Collections

In my experience, the new Groups and Teams, will automatically create hidden "Sites" inside the ROOT site collection.  I'm  learning fast not to put anything in there (the core site you started with) that I need to keep track of.  Having said that, what you are trying to do is incredibly easy once you learn what Teams and Groups are doing and where Microsoft is heading.  To get there in either Azure, or Office 365 Admin center, go to "Groups".  You will see the named group of the site that was created.  You will see people in the Owners and Members Group.  Add yourself to the Owners Group, and consider making the current owners as members instead.  Then try to navigate to the url you found, and you'll be there.  The real culprit here is "how do you prevent people from doing this", and it's vague.  You can disable it entirely (I don't recommend it) following the instructions here https://support.office.com/en-us/article/manage-who-can-create-office-365-groups-4c46c8cb-17d0-44b5-9776-005fced8e618 , but I prefer to not fight where Microsoft is heading, and use this https://support.office.com/en-us/article/office-365-groups-naming-policy-6ceca4d3-cad1-4532-9f0f-d469dfbbb552

Teams, as you know now will replace Skype as the collaborative platform, so I believe it's important to hammer this stuff in advance, because if you disable anything, it will harm your Teams rollout.  Hope this helps.  There's plenty more to it.