Project treble is the initiative to get manufacturers able to update their devices quicker. The idea is that the manufacturer gets its own section of storage for customizations, two sections of storage for the core OS then there's a user section (where your stuff is).
I believe it also includes the dual OS partitions where the update process updates the unused partition while you still use the device then updates that other partition after the boot (so you don't have a device that is unusable during the upgrade process). All of this works VERY well on the Pixel devices - once I press the update button, it goes through the process in the background then I'm prompted to reboot where it reboots to the updated partition then it updates the one that was previously in use.
Oh yeah, here's a fairly lengthy article that I skimmed but should be a good read for a more in-depth explanation -
https://www.androidauthority.com/project-treble-818225/
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