Jamboard is the built-in whiteboard in Google Meet and can also be used as a standalone digital whiteboard. It's not the best one out there - see Whiteboard or Whiteboard FI for other suggestions - but it is integrated. Also, if yours doesn't work, as mine didn't at first, you may need to upgrade your graphics card.
One excellent way to use this is to bring in background images to write on. Since mine wasn't working I didn't save some wonderful links but I'll go back and try to find them now.
How to use the Whiteboard Feature in Windows 10