Chrome moly tube is the tubular “steel” structure commonly used in aircraft construction. “Aluminium” aircraft often have it for engine mounts and the like. Along with stainless steel for firewalls and the like.

Typically chrome moly is used in the internal framework structure, not the other way around.

To say that an aircraft is made of a particular material is not the complete story.

Aircraft are made of a combination of many materials. Depending on the aircrafts intended role, different materials may play a greater role.