I have a M1 dual boot configuration with two installations of MacOS in separate APFS volumes. I recently updated one of them to Monterey. When I boot into the other system running Mojave, it also prompts for an update but tries to download the whole 12something GB update package again. Is it possible to use the update file from the other installation on this one?
After download the installer is located at
/Applications/Install macOS Monterey.app. A copy onto the same filesystem can take advantage of APFS's clone feature and should suffice if mounted from the other installation.