Happy to help! I like studying this stuff, and it's fun to share it when I get the chance.

Honestly, I suspect the "demons torture humans in hell" probably originates from their seeming to want to torture the possessed.

Because in the biblical conception of hell, it's very much not "demons torture humans" it's more like a lake of fire to torture the demons, which unfortunate humans are also thrown into. There's no organization or structure whatsoever. Also, nobody is currently there, humans are just... dead, or in purgatory/Gehenna, a sort of neutral waiting place, waiting to be raised back to life at the end, and sorted then.

Their role biblically seems to be just... acting against God, out of spite for being kicked out, perhaps? They seem to act to tempt humans not to find/love/follow God. Not much is given as to their motivations though, the Biblical authors truly aren't that interested in them, besides as a warning about temptation. A shame, as they're obviously just... fascinating to learn about, but it's not a priority for them to write about.

They also aren't given much credit, either. Rather than the "epic struggle of God vs Satan" we like to characterize, it's more like... Satan and demons are permitted to roam about, but are absolutely beneath God, and can/will eventually be rounded up and thrown out very quickly. They're characterized as accidentally playing a role in Gods plan, and given tentative leash for that reason. Satan apparently is even still allowed to visit heaven, and argues with God? See Job. Him getting locked out of heaven permanently is one of the kickoff moments of Revelation/the biblical apocalypse. Again, not much detail on this relationship, and honestly some of even this much detail is speculation.

The modern conception of "hell" is quite interesting, as it's mostly just imaginative fiction, likely heavily inspired by pagan cultures that merged with Christianity as it spread across the world.