As I am entering the commercial world, I have discovered that there are several types of “real” aluminum baking sheets, Jelly Roll or 12x17-inch in size or so.
On raw aluminum (non coated, non-non-stick), we have pure aluminum sheets, and we have aluminum coated steel! The first costs more and is of superior conductivity and durability than the second!
I have a set of what I thought was pure aluminum, but discovered it is steel with coated aluminum, a magnet sticks to these, as it peels off, too!
To quickly find out what your “real” aluminum pans are made off (including round, square, etc baking pans), try to stick a refrigerator magnet on it. If it sticks then it has steel!