MS Teams actually makes use of WebRTC. That's a standard for VoIP (and similar) via web browsers. Mozilla and Google standardized that a few years ago and implemented it in their browsers, so what Microsoft did, is that they basically shipped a whole Google Chrome to users.
I believe, they did rip some code from Skype for Business for the integration into Windows, though. In the early days, the OS would say that the Teams notifications came from Skype for Business.