is there a sensible way to a data clean room work?

It's a shared environment where you and another company (usually a retailer or a big ad platform) match your customer data against theirs without either side seeing the other's raw records. You upload hashed emails, they do the same, and you get aggregated results back: overlap size, conversion rates by segment, that kind of thing, never individual rows. Ads Data Hub and Amazon Marketing Cloud are the ones most people actually touch. Unless you're spending big with one of those platforms it isn't something a small business needs.