How do fighter jets measure how high they are?
Fighter jets have the same system as commercial aircraft: An altimeter based upon external air pressure. this measures their altitude relative to pre-defined atmospheric conditions. Many will also include a secondary system that relies on GPS (or local equivalent) signals to produce a height above the reference geoid rather than an atmospheric datum.

Regarding your second question - you should only post one question per topic. An answer is difficult as the construction varies significantly between manufacturers. Many pitots are made of steel variants, however.