In the picture below 1 and 2 are the two large air returns (they are at floor level)
2 corresponds to 4 and 5 which are openings in the return duct !
1 is a large register it spans the space between two joists BUT it does not communicate with the air return. It corresponds to 3 which is just a duct that lands at floor level in the basement
It does not make any sense because if you look under the floor from point 1 you can see the air return duct …
I think somebody cut some corners and did not complete the job to connect the air duct that goes down to basement to the main return
here is another angle for that duct that goes between the main floor and the basement without connecting to anything else
Here you can see the return duct that has no opening in it This is the picture of the second joist space where you can see the lip of the duct shoe that starts #3, it goes vertical from that point, while the camera looks horizontally toward the return duct