New to eBikes - where to start?

Hey everyone! I'm totally new to the eBike world and have been lurking here for a bit, soaking it all in. I'm really keen on building my own eBike rather than buying off the shelf, as I love the idea of customizing it exactly to my needs for commuting and weekend trail rides (maybe 20-30 miles round trip on mixed terrain). For those of you who've gone the DIY route, what's the absolute first component or decision you'd recommend focusing on when starting a custom build? Is it the motor, battery, or finding the right donor bike first? Thanks for any guidance!