All the M1 iMac wallpapers are part of the macOS Big Sur 11.3 download. Here's you can easily use them on any Mac.
You can now download macOS 11.3 Big Sur final version for your Mac. Here are the details on the release.
macOS Big Sur 11.3 beta 5 includes mention of two unreleased iMacs. Here are all the details on this.
macOS Big Sur 11.3 update adds game controller emulation for keyboard + mouse on M1 Macs. Here are all the details.