Comment by FuriouslyAdrift

Skype for Business is the VoIP component for Teams, now. Sharepoint is the file service for Teams, too.

Basically, Teams is a front end for a bunch of old Mircosoft cloud services... plus chat. Actually more than one chat as teams channels chat is a separate tech stack from private chat. It used to be much more monlothic and then the Sharepoint people got their hooks into it.