Regarding those 5 folders, opening 1 and then clicking on those white coloured icons produces a window with 'open any files' and immediately another window appears with the heading 'GameCenterAccountsUIPlugin.bundle, and that happens regardless of which folder you open.
Those few hundred files are located HD>Users>shared...over 1300 of them.
Allow me to reinforce Arties warning about messing with anything in the /System folder. It is the quickest way to rendering your system unbootable so unless you have a recent known good clone or like reinstalling your OS frequently — stay out of the /System folder. (Even an administrative account doesn't have sufficient access, except Read, to do much in there anyway.)
The responses you are citing are not normal and before you go any further I would strongly urge you to reboot your Mac — preferably performing a "cold boot" from a powered down condition. You may find a lot of the weird stuff disappears. After rebooting if the "open any files" or "Game Center…" persists try rebuilding the LaunchServices Database. There are several apps that will do this including the freeware
OnyX.
FWIW I just looked in /Users/Shared on three different Macs running High Sierra and the folder was empty on two of them and the only thing in the third is a single folder labeled H&R Block containing a few IRS form update files. (That is the tax program I am using).
