Where can I find sidebar favourites setting files? I would like to have a backup in Dropbox as well as the TotalFinder setting
I don’t know where Finder stores it. It used to be in com.apple.sidebars.plist, but they likely changed it in recent macOS:
https://georgegarside.com/blog/macos/finder-sidebar-preferences-location/
TotalFinder does not interfere with how Finder stores sidebar settings.