I'm new to Sketch and trying to learn as many keyboard shortcuts as possible. I know that you can use tab and shift + tab to navigate up and down through layers OR artboards but I can't see a way to move from a selected artboard into it's inner layers via the keyboard. Is there a keyboard shortcut for this?
When you have the desired artboard selected, hit the return/enter key to activate Edit Mode and get into that artboard's layers. From here you can tab or Shift + tab to navigate as you would on the artboard level.
It might also be of value to know that Esc will navigate back to the parent layer/artboard level.
Here are a couple of great resources for keyboard shortcuts in Sketch: