This article explains some areas regarding the modern Teams experience.
The modern Teams meeting experience starts meetings in separate windows. This feature greatly improves workflow and allows you to use Teams for other tasks whilst still having the full meeting open in another window.
New Navigation Bar
The navigation bar has moved from being bottom centre to top right. You still have all of the same features as before however they are found in a slightly different order.
Microsoft have now added the "Large Gallery" option. This will allow you to have up to 49 meeting participants on the screen at any one time.