eBike technology explained

Hey everyone!

I've been diving deeper into some DIY modifications on my hardtail eMTB lately, and it got me thinking about the core tech that makes these things tick. Specifically, I'm curious about motor types (hub vs. mid-drive), battery chemistry, and controller programming. What's one piece of eBike technology you found particularly fascinating or surprisingly simple/complex when you first learned about it, especially if you've tinkered with it yourself? I'm trying to wrap my head around optimizing my setup for some longer, hilly trail rides.