It depends how they are called
Some people use the terms substrate/bedding/litter all to mean the stuff you put on the bottom of the cage - which I tend to think they all are.
I call the stuff they nest in "nesting material". So there are various types of substrate you can use, but nesting material needs to be safe and something like plain white toilet paper torn into strips. They take it from a pile you put out and take it to the house themselves to build a nest the way they want it to be.
Now you could have paper substrate as well as paper nesting material - and sometimes hamsters will take a bit of that to add to the nest as well. But they do need separate nesting material as well really. Substrate paper is smaller pieces - they like the longer thinner strips of paper for nesting, - our last hamster actually used to carefully weave them into a kind of cocoon!