Resource for understanding Drupal Core functions?
The Examples module is a good way to see "how to do things".  Another useful tool is grep - you can download all of contrib, and search for examples where other developers have implemented the hook or function that interests you.  You can "upgrade" to ack for more powerful searching.
The book "Pro Drupal Development" used to be a good resource, but sadly the 7.x edition is rubbish.