I understand that part. What I'm talking about specifically is part of a bootloader, see for example this page in the documentation of SWUpdate:
https://sbabic.github.io/swupdate/bootloader_interface.htmlBy adding communication between the OS and the bootloader it's possible to implement redundant updates for whole partitions (specifically A/B-updates with a boot counter). U-Boot supports this (depending on the state of the vendor-provided fork better or worse), and Tow-Boot seems to be based on U-Boot.