I want to know if there is some explanation for why the A380 bottom is flat instead of the oval shape, just near the wings.
There are 2 main reasons:
- The wing spars run through that area. If you wanted to keep the wing spar within the oval cross-section, you'd have to install the wing much higher on the fuselage which means the wing spar goes through the passenger cabin.
- Behind the wing spars, the landing gear is stored. The flat bottom provides more internal space.
Another image showing more detail of the structure:
Hobbes' reply has some of the main reasons, but there's some more:
That flat bit is called the "belly fairing", and its functions are:
Avoid some aerodynamically awkward corners that would happen if the wing profile just went straight through the fuselage, with no rounded corners etc. at the intersection. This is particularly useful at the wing trailing edge, to avoid or reduce corrner separation on the wing suction side. This paper shows what that can look like in a generic wing/body configuration, and here's a picture of a design well-known to have an annoying corner separation that could have been prevented with the right kind of fairing/fillet:
Fuselage structure and pressurisation: The fuselage in most passenger aircraft has a circular cross-section because that can deal with pressure differences between the cabin and the outside better. Although it's not usually pressurised around the wing junction below the passenger deck, it's still structurally much easier to just keep using the same circular (kinda elliptic in the case of the A380) sections, and build additional structural elements around it. Of course the wing box and most of the sturcutural wing elements intersect with the tube, but the main fuselage-shaped spars should not be interrupted, as you can see here for example (That's a C919 fuselage mid-section):
As Hobbes has mentioned the whole wing structure needs to stay below the passenger deck, and the A380 has a particularly large and thick wing, so the structure needed to keep the left and right half together is a bit thicker than the space below the passenger deck allows, and the lowest point is the inboard trailing edge -- that's why the belly fairing is so wide and extending below the fuselage in the case of the A380
Also already mentioned by Hobbes: The landing gear. Structurally, it makes sense to attach the gear directly to some of the fuselage spars, and that means that at least the attachment has to be outside the fuselage tube. That takes up space, and that space needs to be smoothly covered by the belly fairing.
You can look at lots of airliner pictures, and they all have some kind of belly fairing, but the A380's is probably the biggest one. That's probably because it has a very long wing chord, a huge wing, and more and bigger landing gears than most other airplanes. Because volume (thus weight) scales with the 3rd power of size, but structural strength only with 2nd power (i.e. cross-section of structural elements), that means it has to be build a bit sturdier and less slender than smaller airplanes. It was also originally planned as the smallest version of a family of aircraft, with one or two versions having longer fuselages -- that makes the belly fairing look even bigger compared to the fuselage.