When you turn on your phone, it boots Android. Obviously. But what if you wanted it to boot a different Android? The first "live bootable" versions of Android are available starting today with Android 10. You can download the so-called Generic System Images (GSIs) right now for ARM64 and x86 platforms.
Google started talking about GSIs earlier this year, and support arrived on Pixel phones with Android Q Beta 4—Google also refers to the feature in Android as "Dynamic System Updates." Using ADB, you can create a temporary system partition on devices with Treble support. It should work best on Pixels currently, though. The generic image run rather like a live bootable Linux distro. You can use the OS for development or testing, and then reboot your phone to get back to the "real" build of Android that's installed permanently.
Google stresses that GSIs are intended for developers—and indeed, Pie only supported GSIs custom-built from AOSP. Now, Google has full Android 10 AOSP builds for ARM and x86 systems, as well as an ARM build with Google services. With official image releases, Dynamic System Updates could eventually have wide-reaching impacts on the way end users experience Android. This is pure speculation, but it may be feasible to run custom ROMs on devices without unlocking the bootloader at some point thanks to GSIs.