Edit: Posted at the same time as Velma
It is a bit confusing. Substrate (sometimes called bedding as well) is the stuff on the floor of the cage. It looks like yours is pine shavings. Pine shavings in the Uk are kiln dried by law which removes a lot of the phenols, but it's always good to check if it says kiln dried and "dust extracted" on the bag. Strong scents and dust are bad for hammies who can easily get respiratory problems. But I'm sure yours will be kiln dried and dust extracted.
Nesting material (also sometimes called bedding!) is the paper strips for building their nest. But you can have paper substrate as well. That's the Fitch. I love the Fitch and our hammy loved it. It works out cheap because you buy it in bulk. Hamsters will sometimes use some of that in their nest as well as the paper strips, but they can weave the paper strips to hold the nest together better.
If you need some bedding (substrate) then getting a bag of Fitch should last you quite a while. The more you put in the cage, the less often you need to do a full cage clean so it works out quite economical. Especially if using a litter tray.
I get this 10kg bale because it's not too big to store indoors.
https://www.fitchrecycling.co.uk/ani...-delivery.html
If you've got a spare bedroom or something it works out even cheaper to buy the 20kg bale which is only a few pounds more. It does need to be stored indoors though or it can get damp and go off.
https://www.fitchrecycling.co.uk/ani...bale-20kg.html
My 10kg bales come in oversized plastic bags, inside an outer plastic bag, so I find you can squish it into different shapes to store it with the bag having space in to squish it without the stuff falling out. So I've had it squished tall and thin to fit between wardrobe and the wall, short and fat to fit under a bedroom chair. But latterly I just had it in the corner of a room with a throw over it so it looked like a small beanbag chair