I took the clamshell spare tire holder off a couple of my vans and one
of them has been sitting outside ever since.

And not rusting.

Which makes me curious. What is it made of? I can't imagine that it is
aluminum. Seems way too thin for that. yet it's not standard steel, as
the bare metal would have rusted a long time ago.

It's relatively light weight, but also fairly thin.

A wandering mind on a Fryday wants to know.