eBike technology explained

Hey everyone!

Been lurking for a bit and finally decided to jump in. I'm really curious about the tech behind our custom builds, especially when it comes to integrating different components. For those of you who've built your own eBikes from the ground up, what's one piece of eBike technology that you found surprisingly complex or surprisingly simple to work with? I'm thinking about motor types, controller programming, or even just battery management systems. Would love to hear your experiences and any "aha!" moments you had while piecing things together!