I can’t imagine they use Teams internally.
Most groups in Microsoft use the Microsoft stack as it's best supported and already compliant and works pretty well - so most groups use teams, office, Azure etc.
Groups are free to use other tech and may for example use zoom+slack (some groups do) or GSuite. This is also true for technology (e.g. a team can use yarn and flow instead of npm and TypeScript). It's just very rare they do.
Source: msft employee