What did you only learn after 100-200+ hours?

I'll lead with, I only learned that I could use the re-root tool to make sub assemblies, so my first and only sub assembly is my Ventura interplanetary stage/launch booster, and I've put nearly 430 hours in the game as of logging out that day. What have you learned, or even only recently learned after being in the game 100, 200 some odd hours, and how dumb (or smart) did you feel when you finally discovered it?