As a presenter (see below for roles and permissions), when you share an uploaded presentation (see above), you can add annotations (lines, geometric shapes, arrows, text, notes) to it. You will see an annotation bar at the bottom for this. At the top under "Styles" you can select the color, thickness, line type, size and font of the annotations. If you click on the cursor ("Select") in the annotation bar, you can move existing annotations or otherwise change them.
By default, only the person with the "Presenter" role can make annotations. However, the presenter can activate multi-user mode using a button in the row below the annotation bar.
Now everyone can use the whiteboard tools - until the presenter deactivates the multi-user mode again. As a presenter, you can also give individual participants access to the whiteboard by selecting them in the participant list and choosing the corresponding option. Individual participants can also be denied access to the whiteboard.
By the way, the annotations stay on the slide. When you move to the next slide, they are gone. When you move back, they are there again.