Modern team site - How to create modern ROOT and SUB site

My 2 cents here:
(1) A modern team site is behind the scenes a site collection so the idea of creating a subsite as a modern team site is not correct. First you create the modern team site and then you can create as many subistes, lists and libraries as you want in the modern team site.
(2) As far as I understand, current rollout (not finished yet) will allow you to create modern team sites from the SharePoint landing page. In the future, I expect we will be able to create also modern team sites from the SPO Admin Center.
(3) Modern Team Sites are being released to First Release, what means we can consider they are still in preview so that's could be the reason why you are not seeing the site in the SPO Admin Center. I'm pretty sure that currently they are still being created as hidden site collections (As happens when you create an Office 365 Group and you provision the Group site)