Yup - you got it ... those are self-serve, coin-operated vacuum cleaners. There will be instructions on them as to how many quarters you need for how many minutes (or they may take a credit card). They have wide hoses to help prevent clogging.
"I guess maybe that's what the "detail" part of their title is..."
Detailing is additional work performed by humans Some places have "express" detailing which is done "while you wait" where an employee will, for example, vacuum all of the interior (floors, seats, floor mats, etc.); clean all interior windows; clean all interior surfaces (back window ledge, front dashboard / windshield area), and dashboard itself, as well as cupholders, etc. -- may, or may not apply a cleaner/protectant like Armor All*.
While-you-wait detailing (if available) will typically take 10-15 minutes, depending on staffing, and the number of customers. Tipping the detailers a few dollars is appreciated (if there are 2 people working on your car and you don't have smaller bills, you can tip one and ask him to share it
Other facilities may offer detailing but will ask you to leave your car ... (maybe for a hour.. don't know). While yet "full detailing" is offered at some facilities -- which is definitely a "drop off and pick up later" scenario, and some/many offer appointments. "Full detailing" includes everything mentioned above, and can include exterior hand-waxing, wheel cover cleaning, and polish, tire cleaning, interior fabric cleaning, floor mat cleaning, protective coatings as needed, some will paint the rear window ledge if it's become faded, or marked. The trunk is also vacuumed and all interior surfaces are cleaned. The dashboard area receives particular attention, with all surfaces being cleaned and perhaps protected (waxed)... it is not unusual for the Q-tips to come out at his point to clean the surfaces of the air vents and other hard-to-get-to dust collectors
Most, if not all facilities offering detailing have a menu (which may also be on their website) offering different "levels" of services, -- the old "Bronze", "Silver" "Gold" "Platinum" depending on what you want to be done, and wanna pay.
If you're going for one of the more involved detailing options, plan ahead and take out any extra stuff you may have in the car, trunk, glovebox, before you go
A car just seems to perform and run better that's just been detailed!
*some car washes have vending machines that dispense single-use quantities of Armor All, and other products for self-serve "detail cleaning".
*Also what Bing and Volvo_V70 said while I was typing