How do you open Dev tools in the Teams app?

On Windows: Quickly left-click on the tray icon a lot, you’ll then get additional options in its context menu.

I checked again and it indeed appears that at least part of the application was rewritten in a different framework. Most of it is still Angular.js though.

I don't know about the app, but you can open the same thing in a normal browser:

https://teams.microsoft.com/