Using AppleScript to sync iPhone in Catalina Prior to Catalina I used a script to trigger a sync to my iPhone and iPad - similar to:

tell application "iTunes"
    tell every source whose kind is iPod to update
end tell


Catalina removed iTunes and iOS devices are now synced using Finder.

What do I need to call in order to trigger a sync in Finder?
I expanded on dyindude's answer to make a single script that opens Finder, navigates to your device in the sidebar, waits for the sync button to appear, and clicks it. This assume's your device is named iPhone:

tell application "Finder" to open ("/" as POSIX file)

tell application "System Events" to tell outline 1 of scroll area 1 of splitter group 1 of window 1 of application process "Finder"
    set theElements to first UI element of every row whose name is "iPhone"
    repeat with e in theElements
        try
            if name of e is "iPhone" then
                tell e to perform action "AXOpen"
                exit repeat
            end if
        end try
    end repeat
end tell

tell application "System Events" to tell application process "Finder"
    repeat until button "Sync" of splitter group 1 of splitter group 1 of window "iPhone" exists
    end repeat

    click button "Sync" of splitter group 1 of splitter group 1 of window "iPhone"
end tell