As Apple still hasn’t provided any detailed information about security or other updates which were provided in its recent Supplemental Update for MacBook Pro 2018 models, the only answer is to examine the contents of the installer package using Suspicious Package.
First, this reveals a complete set of EFI firmware updaters for other Mac models, but none for any MacBookPro models more recent than the 14,3, which seems very odd. Even wierder is that fact that for several older models – iMac17,1, iMac18,1 and 18,3, MacBook9,1, MacBook10,1, MacBookPro13,1, MacBookPro13,2 and 13,3, MacBookPro14,1, 14,2, and 14,3 – the version numbers provided with this Supplemental Update are one major version lower/earlier than those supplied with the High Sierra 10.13.6 update!
Only two apps are replaced: Safari (version 11.1.2 still), and Bluetooth File Exchange (6.0.7f11). CoreServices apps replaced include RAID Utility, System Image Utility, and MRT (still version 1.35). The Bluetooth and TouchID panes are also replaced.
A few tools in /bin and /sbin are replaced, including sync, wait4path, dmesg, dynamic_pager, fstyp_hfs, halt, ifconfig, mount_smbfs, mpioutil, nologin, ping, ping6, reboot, route, rtsol, and shutdown. A lot of tools in /usr/bin, /usr/sbin, and /usr/libexec are replaced.
As has so often been the case in recent updates, most or all Apple extensions (KEXTs) are replaced. APFS remains at version 748.51.0, the same as in 10.13.4-6.
Altogether, nearly 13,000 items are installed, occupying 2.13 GB on disk.