Go to your Team’s setting(Desktop Application).
Enable the new meeting experience manually by checking the icon as shown in the image below. By doing so and restarting the application, the multi-windows mode will be enabled as discussed officially here by Microsoft.
7X7 view is supported only in pop-up windows enabled mode as of August 2020 update.
Go to meeting options to switch your view accordingly as shown in the image below. To enable a 7X7 view in a team, you need to select a Large Gallery(Preview) option there just above the “Choose Virtual Background” option shown below.
Your view will be changed.