To put an item inside, carry the item you want to add, click on the Backpack, and then use the left stick to Stash the item. (On PC, just hold E, and use your mouse to physically move the item to ...

So I'm told - as you are - that backpacks are in. Been around that block before, though, so here is my caveat. Whatever you do, do not attempt to put in a backpack: -- Any sandwich containing ...