In this video, I try to illustrate why there seems to be this misconception that Office 365 Groups (and as such, Teams and modern SharePoint Team Sites, based on Groups) can contain Security Groups (or any other types of groups).
They can’t. So stop that train of thought… 🙂
Microsoft Teams DOES allow you to select a group when picking members for your new team to help you identify a lot of people at once … but from there, those people are added individually as members to your Team (and subsequently, Office 365 Group) and that is where the Security Group relationship ends…
As always, I hope this helps someone.