Most in and around passenger cabins aren't that mysterious. Some are crew access/emergency stairs, some are steward supply rooms.
Mid aft to aft, the void on all decks includes the exhaust stacks.
In this case, the proximity to the Lotus Pool would hint plumbing systems and or pool/Lotus Spa supplies.
No specific website, but I will always do a search for my cabin before booking it. If someone has had a good or bad experience, they will usually tell you the pros and cons, unless they are a only-pro influencer. Most ship cabins more than a couple of years old have a Youtube video done, and I find that more useful than the deckplan site these days.