Actually I didn’t write that quite correctly, the black “hot” wire is the only one alternating from 0 to positive 120 volts then back to 0 then down to negative 120 volts... but that’s as measured between the hot and the neutral so it can appear as if the neutral is doing the same thing.
And really, it peaks around 170 volts, the common 120 number is an average. It’s called RMS but you don’t need to know any of that biz. Forget all this, you don’t need to know any of it, I just know there’s electrical guys cringing at how far I crunched it down haha.