Hi,
I would like to understand how the metal exposure is defined. For example, the highlighted white area in the picture are the pads that I would like to let the metal exposed on them for soldering (They are defined as pad in the layout editor). But I am also confused because the pad and the wire in the layout editor both are drawn using "TopLayer". How could the process tell the difference between the pad and the wire? (Pad with metal exposure and wire without metal exposure)? Thanks.
Brand new interactions and interfaces. 
Easy to use and quick to get started. 
the exposure is controlled by solder layer, for make it easy, all of the Pads will be added the solder layer automatic, so you don't to do any thing,
If you want to control, to you can make the solder layer visibility, and to add.
You can use the photo viewer to see how your PCB looks like.
https://easyeda.com/Doc/Tutorial/PCB.htm#Photo-View