eBike technology explained

Hey everyone!

I've been riding my hardtail eMTB for about a year now, mostly on forest trails and some gnarly climbs, and I'm constantly amazed by how much technology is packed into these bikes. I'm really curious about the nitty-gritty of how different motor types (hub vs. mid-drive) and battery chemistries actually impact performance and longevity, especially for those of us who like to tinker. For DIYers, what's one piece of eBike tech you think is most crucial to understand before attempting any significant modifications, and why? I'm particularly interested in hearing about controller programming or sensor types!