Last year,
Google speculated it might be possible for Android devices to boot a GSI (Generic System Image)
without needing to unlock the bootloader and flash it. That functionality was
shown off as working just recently at this year's Google I/O developer conference, under the name "Dynamic System Updates," or DSU. (It's a poor name, since DSU effectively lets you live boot into other Android images.) We were told the functionality would eventually make its way to Android Q, and as of Beta 4, it has.
Dynamic System Updates work by loading an image (such as a GSI) into a new "dynamic" partition which Android can boot from. This allows for the stock system image to be preserved while you're running an entirely different image, sort of like running a live image of Linux off a thumb drive, if you've ever done that.
Google's documentation for DSU states that the feature has been enabled on the Pixel 3 and Pixel 3 XL as of the Android 4 Beta release, though it's a bit complicated to get it working. You'll need to run a specific ADB command to enable the feature, and further invoke it over an ADB shell as well.
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