Why does this wall in the basement have so many vents

If the furnace and water heater are gas powered, it would be to provide adequate airflow for their combustion so that you don't get carbon monoxide forming in the utility room.

Here's a link with some information. An excerpt:

The heating appliances are fully isolated from the living space with partitions. Figure 1 provides two vents to the outside, one within a foot of the ceiling and one within a foot of the floor. If vertical ducts are used to bring combustion air to the appliance each vent should be sized at one square inch of free vent area per 4000 BTUH of the appliance input rating.

(This is based on a half-remembered long-ago conversation with a service person working on my furnace and heater, which are in the center of my mostly-unfinished basement: I mentioned framing in a utility room around them and he said not to for that reason).

You want a second return in the ceiling of the top floor for cooling. Otherwise, all the hot air will collect upstairs and you'll have a very difficult time cooling the space. Any air return I've ever seen, that goes between floors, involved a wall being built out. Usually it's hard to tell because it's the back side of a bathtub that looks like it's built out for the bathtub to fit, or a closet that's a bit narrower. You may get lucky and already have space for it behind one of your walls, but that would require looking over a detailed floor plan to see.