The main meeting controls are along the bottom.  Going from left to right: 

Audio settings, Video settings, Screen Sharing, Record the meeting, Raise hand/Reactions, More options (Here you can change meeting options and cast to a video system), End meeting, Apps, Participant List, Chat window, and panel options. 

These controls, as well as others are available through the top menu options too.

Screen Sharing

To share your screen, click on Share, then choose either the screen or application you wish to share.  

Once sharing, a different control bar will appear at the top of the screen being shared.  These options include being able to pause the sharing, assign another participant control of your mouse/keyboard, and ability to annotate on the screen.

Passing Presenter

To pass the presenting rights to another participant, you can click the Participant icon to open the list of participants.  From there, you can hover over their user icon and a little Webex icon will appear.  Clicking this will prompt you to confirm making the participant the presenter.

Presenter and Host rights can also be given by right clicking on the participant, choosing Change Role To and then selecting either Presenter or Host.  

Whiteboard

From the controls bar at the bottom, click the More options button and then Start new whiteboard.

This opens a whiteboard in the meeting window that allows for drawing, text, shapes, multiple pages, and participant collaboration.  In order to allow others to edit the whiteboard, the participant must be using the downloaded application, and not through a browser.   They must also be given permission by the presenter by opening the Participant list, right clicking on their name, and checking the Allow to Annotate option.