I’m not sure that it’s correct to say Ethernet uses modulation at all. Modulation is used to describe transmitting data as waves such as those used by radio or (closer to what you’re asking) modems.
Ethernet transmits bits one at a time using something called “Manchester encoding.” Manchester encoding works by looking at the transition of the signal from 1 to 0 representing 0 and 0 to 1 representing 1. The “preamble” is used to fix the clocks on the source and destination host prior to the transmission of actual data.
Given the scheme uses transitions as a means to encode the bits I suppose you could argue it uses phase modulation but I’d argue you’re wrong. 🙂