Differences between old site collections and modern ones

Is there any place I can read about differences between older legacy site collections (around 2013-2014) and the site collection that Office 365 Groups creates its sites in? I see differences in the way privileges are grouped in the permission levels, and I'm trying to identify if they're changes that came with successive feature improvements to O365, or if they were configured to be that way by local tenant admins.

Thanks,

Anne