As you Microsoft Teams is for collaboration, meeting and/or calling services.
Well a new meeting and calling experience is under way.
This ne new experience will open a new window for your meeting and/or call while keeping the main Teams window available, letting you free to continue to use your main Teams window for collaboration or instant messaging.
The first things will be to turn on the new experience and then restart your Windows Teams client.
NOTE the new experience will become the default experience sometime after August and end-users will no longer be able to enable/disable it
To turn on the new experience, open the Teams client and access the Settings menu
Then under the General section you should see the option Turn on new meeting experience (if not your client has not yet got the update)
Restart your Teams client
Now you are using the new experience which is opening a new window while you are on meeting.