Most of the time you should be working on both, sometimes you do shallow work, sometimes you do deep work and you naturally build a deep understanding of the pi

Most of the time you should be working on both, sometimes you do shallow work, sometimes you do deep work and you naturally build a deep understanding of the pieces of the stack you mostly work with. I find it hard to believe anyone does serious development work without some deep understanding of a piece of what they work on, the folks I've met that did that didn't last multiple review cycles.