How can I determine plist changes while changing System Preferences to automate them next time?
Apple stores most relevant preferences in a database now, so your best bet is to ignore plist files on the filesystem and focus on the output of defaults read com.apple.whatever for the things you change. Unfortunately, there isn't a good Rosetta Stone that says if you are in preference X - look for changes in these seven preference domains. Where Apple stores iCloud or security information doesn't map to each visual icon in system preferences so you'll need to issue more than one defaults read to capture the entire state of most preference panes since most panes write to more than one setting domain.

Alternatively, you could just use MDM and configuration profiles and scripts to make all changes so that you just push the changes to all machines from your management tool and skip the whole reverse engineering process entirely.


https://support.apple.com/profile-manager
https://developer.apple.com/library/archive/documentation/Miscellaneous/Reference/MobileDeviceManagementProtocolRef/3-MDM_Protocol/MDM_Protocol.html
https://www.jamf.com
https://www.jamf.com/jamf-nation/discussions/15362/learn-mcx-and-configuration-profile


In the past MCX was the path to managing enterprise configurations and that's the equivalent of your wanting to look at the .plist files that stored those settings initially at the dawn of OS X - but the current system of configuration profiles, scripted changes and MDM hooks has left MCX and .plist to just the dusty corners that haven't been updated for the new central database to store user and system defaults.