The systems I've seen vary, from aircon ducting (as you say) to fans under the seat squab that just suck up ambient air from the rear footwells.
In all cases there are then channels through the seat up to perforated leather, and your bum. Opinions on how well they work vary enormously, for example, despite both TB and M'Lud running Jaaaaags of similar vintage, their experiences are poles apart.
I've only experienced them in a new Renault Velsatis (not sure we get the model that has them in the UK) and tbh I couldn't work out if the buttons were even connected to anything or whether they just operated an LED on the dash. That said, being a Renault, they were probably broken...
Back on topic, at least it seems I'm not paying over the odds, which is all I'm bothered about and come to think of it, Formula one did a decent job on the mig's a/c 2yrs ago.