Right, I think the idea is that he had hidden cameras that would video whatever they were doing while staying at his places. Whether that be doing things with the underage victims, or drugs, or whatever.
The DOJ memo says that their case file on Epstein includes videos of child abuse. Those COULD possibly just videos of him abusing them, or they could be his blackmail material. We don't know and they're not saying. So just to put it out there, while I kinda don't think I believe that there is a "client list" as such for the Trump DOJ to suppress, it's totally possible that there are identifiable people on those videos that the DOJ has been pressured or ordered to NOT go after. So like, there still totally could be a cover-up going on there.
Another thing to consider: the people who visited Epstein at his houses were his friends and people he wanted to like and support him. I can see him feeling a need to create dirt on them as a "just in case" thing, but the fastest way to destroy trust or ruin a friendship would be to tell someone out loud that you've got dirt on them. If they know that they were abusing underage girls, and you know it, and they know you know it, you don't even need an actual video to keep them in line, just the threat that one might exist is probably enough.