I have several cards in corkboard all under one card. How do I expand them so I see all the cards ?
If you are looking for something like a “flattened mode” that shows both the parent cards with all of its children cards after it, there is nothing like that in Scrivener. A few conceptual ideas were tossed around in the past for including something like this, but no good aesthetically appealing and functional ideas came of it. You can of course view the contents of each section by clicking on them in the Binder one by one.
That’s the bad news. The good news is that Outliner, due to its format, does allow you to view the entire structure of your book all at once. You can use Option-click on the arrow (or Option-LeftArrow on the keyboard) to expand entire sections all the way down to the bottom. Since Outliner columns can be customised to show all of the information available in index card mode (actually to a greater degree since Synopsis is not truncated due to card size), it is from an information standpoint, just as good. Of course if you prefer the aesthetic appeal of the index card view, that won’t be satisfied by Outliner mode.
Actually you can look at a folder flattened, using View > Index Cards as Stack (though you can’t drag and drop in this mode because it is flattened).
Best,
Keith
Oh, true! I keep forgetting that was added. 
Is this something in 1.12? I can’t find it in 1.11.
I think it was added in 1.10, but it might have been 1.11. You can find the function in View>Open as Index Card Stack. There is a keyboard shortcut for doing this in the main editor. Also, it is available as a contextual menu item when right-clicking on a group in the Binder.