I frequently hear gate agents announce that they are looking for volunteers to check their carry on bags. After the flight, there is sometimes a line in the jet bridge, supposedly waiting for their carry on. So I assume the checked carry on bags are not stored in the main luggage area of the plane. Is that true? On the other hand, the other day my carry on got checked, and I had to receive it in the baggage claim area. Does it depend on the plane where the bags are stored?
These are typically two different things (at least for United).
Bags checked by volunteers will be checked through the final destination, get a regular baggage label, and are picked up on the carousel of your final destination. That typically happens when the overhead bins are full.
Some smaller planes can't accommodate regularly sized carry on at all. In these case, your bags get a "green tag" for identification purposes and they get dropped off at the end of the jet bridge or on a cart at the bottom of the stairs. These bags get returned at the jet bridge (or stairs) at the end of the flight regardless whether you are connecting or not.
In both cases, the bag travels in the cargo hold.
Checked-in carry-on luggage will be stored in tha aircraft's hold, just like any other checked-in luggage. Where one retrieves it from will depend on the airport and airline.