In Apple terms, a custom build, also called CTO (Custom to Order), means your MacBook is made specifically for you. Apple offers a few “base models” they expect most people to order, and everything else (like extra storage or RAM) is considered a custom build.
Custom builds ship directly from Apple’s factory in China and typically take around 4 weeks to arrive (so, more than the base models!).
As soon as your MacBook lands in the U.S., you’ll automatically receive a UPS tracking number via email with the delivery details.