Emu is just an abbreviation for 'emulator', in this case referring to some software that just makes your PC system seem like another to other applications. Steam emulators are used to run steam games independent of steam itself. The emulator handles all interactions between steam and the game (i.e. the steam API calls), so one doesn't need steam anymore to play steam games. One less launcher off your system and no need to be online to play any of your games. If one has the steam version of GTAV, one can use it to rid themselves of steam. The social club emu I used is so I don't need the rockstar launcher to play; a lot of pirated GTAV releases rely on this to essentially bypass any license checks done by the rockstar launcher, but I had a legit copy from EpicGames, I just didn't want to have 2 launchers (epic and rock*) to play GTAV, so I used the social club emu - ridding my need of them. As I said, doing this will make RPH crash on load, so one has to unfortunately use a bajillion launchers to make RPH work with GTA V.
I should have mentioned that there are multiple ways of 'reinstalling Windows' per say:
https://support.microsoft.com/en-gb/windows/reinstall-windows-d8369486-3e33-7d9c-dccc-859e2b022fc7
^ As explained above, 'Reset Windows 10', 'Reinstall Windows 10 using installation media' and 'Clean install of Windows 10 using installation media' are all options, but only the latter is enough to fix whatever system files windows-defender-disabling scripts may have botched up. If you haven't ran any such scripts you won't need to do this. Though you can try the second option 'Reinstall Windows 10 using installation media' as it will keep all of your installed applications and personal files, only modifying Windows system files.