That I’m not sure, I know the premise is that data is read and written magnetically, which would lead me to believe there is some kind of ferrous metal in there somewhere, but I couldn’t tell you the actual composition of the platters.

In principle yes but I never managed to shatter or break a disc platter… But then I never had a sledgehammer

It depends.

With tearing apart drives over the years, 3.5" always used aluminum platters, while the 2.5" ones used glass. (With greater data density and higher speeds however, this may have changed)