First of all does the centre wing box form part of the fuselage as it's in the centre of the fuselage? Secondly how does the wing spars connect to this centre wing box to hold everything together?
Transport airplanes, in general, use one-piece wings with no joints between sections. The "wing box" is the central structural element for the entire wing, made up of front and back spar webs, and upper and lower planks, which make the 4 sides of the box. Leading edges and trailing edges/control surfaces are tacked on to the front and back walls of the box.
The upper and lower planks (machined skins) are the primary structural members in bending, supported by the spar webs to resist shear loads and the wing ribs to support the planks themselves against buckling. You could say that the entire upper and lower skins or planks are the spar caps.
The "center wing box" is the middle section of the overall wing box, but that just describes the center part of a very long box. The fuselage will simply sit on top of it, with a square cutout in the bottom to fit over the wing, with fairings all around the connection to make an nice external transition for the air flow.
The fuselage will be integrated into the wing box structurally with some sort of connecting links joining fuselage frames to the box, and may also have fuselage skins attached directly to the wing box. Sometimes the cutout in the fuselage has its own pressure bulkhead forming a box outside the wing box and you can simply unbolt the fuselage from the wing and lift it off, and sometimes the wing box itself is made part of the pressure hull, requiring lots of rivets to be drilled out to separate them.
If the airplane has wings that attach to each side of the fuselage with big spar fittings and bolts, the center wing box will be an integrated part of the fuselage, what you would call a structural pass-through.