New to eBikes - where to start?

Hey everyone!

Super excited to finally dive into the eBike world, and I'm really leaning towards a custom build rather than a pre-built. I'm looking to put together something that can handle a mix of paved commutes (about 15 miles round trip) and some light gravel trails on the weekends. My main question for you seasoned builders is: where do you even begin when planning a custom build? Do you start with the frame, the motor, or the battery? Any tips for a newbie trying to figure out the best component compatibility would be awesome!