In a Teams meeting select “Open Share Tray”

On the far right is the Whiteboard option.

Select whether you want only you to be able to use it, or all participants.

Once it opens use the pens and notes to collaboratively work

Note that the eraser is brutal.

In theory it saves in the conversation… in practice maybe not… but it should save them to https://whiteboard.microsoft.com/
You can export the image created on the whiteboard as a PNG, select the settings cog top right, and export as png.