The stem and core is almost always made from this material:
Galvanised steel has been treated to avoid rusting. It is coated with zinc, which binds to the steel creating a non-rusting layer. The zinc layer is also more durable so it doesn’t get scratched so easily and looks new for longer.
The galvanised steel wire can be bent without breaking so it can be twisted together to form the stem and core of a pipe-cleaning brush.