Apparently the new ARM Macs use ipsw files to restore, just like iOS devices. Interesting.
Download macOS IPSW files for Developer Transition Kit
ipsw.me
This is going to be an interesting next few weeks as we learn all about how the platform works.
This worries me a bit, in that I wonder if Apple is going to do signing on these releases, and stop signing earlier releases so we can't downgrade easily. Being able to downgrade MacOS is super-important for some users. It's one of the few things I hate about iOS; for example I wish I could put iOS 6 on my old iPhone 5 for nostalgic reasons but I can't because Apple is stubborn.