If your phone is in A / B partition format, there are two virtual hard drives, each containing a copy of Android. After each reboot, it chooses one partition to boot and the other remains inactive. The inactive partition can be overwritten with a new copy of the operating system and does not affect the active one. So next time you reboot it will just switch partition and it's like updating it immediately.
Google added this feature to Android in 7.0 Nougat, but they never made it mandatory. Unfortunately, most manufacturers didn't take it over, probably because they didn't want to lose advertised storage space for a second copy of the operating system. They finally make it a requirement for phones that come with Android 1
Checking for A / B partitions  The easiest way to do this is to install Treble Check by developer KevinT. Thanks to XDA for the find, but this app is free to install on the Play Store and can tell you in seconds if your device has duplicated partitions.
Once installed, just open it and see what's under "Seamless System" Updates. "If it says 'Supported', congratulations! If not, at least you know your next phone will have the feature if it ships with Android 11 or higher.