O365 Group & Its associate site?

Hello Forum,

"Every Office 365 Group gets a modern SharePoint team site", This a famous statement we heard many times, and many of us love it.

My colleague has created a group and invited me into it as a member, and I do see it already in my Outlook and I can access its modern team site which its URL is: https://TenantName.sharepoint.com/sites/GroupName/

My questions are:

1) I am a global administrator in our Office 365 tenant and I have full control on all our services. But, When I go to: SharePoint admin center > Site Collections > I did NOT find the Group's Team Site listed there - Why? The URL seems to be in my tenant, But, Where does it reside?

2) How can get hold of this site and reach it i.e. access it and manage it ?

3) Whenever I access this Group's Team site, I am ALWAYS keep on redirecting me to:

https://TenantName.sharepoint.com/sites/GroupName/Shared%20Documents/Forms/AllItems.aspx

Why cannot I go to other URLs of the site like: 

https://TenantName.sharepoint.com/sites/GroupName/_layouts/15/settings.aspx
https://TenantName.sharepoint.com/sites/GroupName/_layouts/15/viewlsts.aspx

Of course, I wanna access these pages to be able to Add page and try to learn.

Please, Help me to understand more about the group's site !