50 amp breakers are dual voltage, 120-240-120 Depending on which wires you draw power off of..
The TT 30 outlet has, clearly printed upon it "MAX 125 Volts" And is not to be used for 240 volt service.. Still many do wire them that way. and live to regret it. Because they are 120 volt.
15 and 15/20 and 20 amp outlets are,, as you would expect, normally 120 volt (Though I do know of one that was or may well still be an exception).
But yes, There are many threads around here about folks who had an electrican install a TT-30.... Wrong
Even one from a Rver do did it himself
I did mine right.