Update your client:
To use Large Gallery, Together Mode or Focus you must be on the latest version of the Teams client. Click your picture in the upper-right corner and select "Check for Updates"

Teams will check for the updates in the background. Give it about 5 minutes to see if there is a new version. You may need to restart your Teams client after this is complete.
Turn on the "New Meeting Experience"
Currently, you must manually enable the "New meeting Experience" to see Large Gallery and Together Mode. Click your profile picture, and select Settings.
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Select your profile picture at the top right of Teams, then Settings.
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On the General tab under Application, select Turn on new meeting experience. Then close Settings (select the X).
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Finally, restart Teams so the changes take effect.
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On Windows, right-click the Teams icon in the taskbar > Quit. Then open Teams again.
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On Mac, Ctrl+click the Teams icon > Quit. Then open Teams again.
What changes:
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Meeting and call controls are at the top of the meeting or call window, instead of the bottom. They’re docked there so you always have access to them (without jiggling your mouse) and they’re out of the way of the content and video that’s being shared.
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Large gallery view lets you see up to 49 video streams at once. This view is available when there are 10 or more attendees who are sharing video. To switch to large gallery, go to More options (…) in the meeting controls > Large gallery.
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Together mode lets you feel like you’re in the same shared space with everyone in the meeting. It’s in the same menu as Large gallery, just below it.
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Meeting notes are now taken directly on the meeting’s Meeting notes tab in the main Teams window. To access or take notes from within the meeting window, select More options (…) > Show meeting notes and you’ll see the usual side pane come up. In the pane, there is a Take notes button that brings you to the Meeting notes tab.