Housed in the main gearbox. Other than the ratios being less than 1.00, they're the same as the rest of the gears in the transmission. I don't know if any current cars use a seperate overdrive unit any more.
One thing I haven't seen in an automatic tranny lately is a "lock up" clutch. Once a car was cruising, the "lock up" clutch would engage, directly connecting the engine to the driveshaft so a 1.00 ratio. The advtange was better gas milage since no energy was consumed through via the fluid "clutch". The "lock up" clutch would often act as a 4th gear in an otherwise 3 speed tranny, where the 3rd gear ratio would be > 1.00.