Would it be immature to ask my PhD advisor for more background for a paper they've asked me to help with?
This is a fantastic question to ask. It's where your advisor's expertise really has the chance to make a difference, and it's much more interesting to answer than &quot;Why does line 4 follow from line 3&quot;? Long term it's your hope to understand the big picture as well.
The best time to ask questions like this is after you've first taken a stab at it yourself: you'll be in a much better position to understand the answers! As you've done this already, I'd encourage you to proceed.