UTC isn’t a mystery. There are various kinds of UT, depending on what corrections are applied to the raw observations: UT0, UT1, UT2. UT1 is the standard versio

UTC isn’t a mystery. There are various kinds of UT, depending on what corrections are applied to the raw observations: UT0, UT1, UT2. UT1 is the standard version of mean solar time, though GMT (when it was still a thing) used to be more like UT2. UTC is named according to the same scheme: Universal Time with a suffix denoting which kind of UT.