Basic questions for onUpdate and a background question

For Reference: Toybox.WatchUi.View (garmin.com)

The example shows in the "OnUpdate()" function it calls View.onUpdate(dc);.  What does it mean and is it required?

If we are putting values into labels or creating bitmaps/shapes it seems label values go before it and drawings go after it..  but I'm not sure why.

Another question is setting the background color from the settings/properties file.  Where in relation to the label inputs, bitmap drawings, and calling onUpdate do I set the background color?  This is all very confusing to me on why it's SO particular you get it exactly right.  Unfortunately, in this case I can't even guess what the right order is.  Putting the background color change after onUpdate changes the color but nothing else shows up.  If I put it before then it doesn't change the color at all.