SharePoint Online: What Happens to Sub Sites if You Delete the Parent Site?

There is nothing "mini" about the site collections Groups get! From a feature perspective, they are the same as the site collections provisioned by IT - but they get a modern SharePoint team site. If your teams are able to provision their own Groups, then it is possible that what your IT department has provisioned for you might be a classic "publishing" site, which is oriented towards communication rather than collaboration (think small number of authors and large number of readers). If your goal in creating sub-sites is to create a site for each of your teams to collaborate, then the best "future proof" approach is to configure these sites the way your PMs are doing - as Office 365 Groups (which then allows the teams to use Microsoft Teams). You can "connect" your individual PM sites in a variety of ways. Depending on how many teams you manage, you might find the quickest "no code" way is to create a hyperlink to the team sites on a page in your "parent" site. But you need to think about what outcomes you are going for as well. If the goal is to be more aware of what is happening in these projects, then you have two additional good options "out of the box." (You can do all these things - they are not mutually exclusive.) First, assuming that you Follow these sites, SharePoint home provides a great way to see the most recent activity in these sites from a single "front door." Second, if you create an expectation that your PMs will post major accomplishments as news pages on their team sites, SharePoint home will also show you a "roll up" of the news posts from the sites you are following - both online and in the mobile app. This second approach requires establishing some conventions and norms for your teams - which is always a good idea. (I have written several blog posts about this. Here is the most recent: http://www.networkworld.com/article/3179768/cloud-computing/10-tips-to-get-started-with-microsoft-teams.html) All of this being said, it's really hard to give specific guidance without knowing exactly what you are trying to accomplish. Hopefully, this will help!