When we refer to the solar system or galaxies as being "flat," we are describing their overall shape. This flatness typically refers to the configuration of objects within these systems lying mostly in a disc-like or spiral structure. For example, the planets in our solar system orbit the Sun in a roughly flat plane known as the ecliptic, and galaxies often exhibit a flattened, disk-shaped structure with stars, gas, and dust arranged in a spiral pattern.
However, it's important to note that the concept of "above" and "underneath" doesn't necessarily apply to the larger structure of the universe beyond these systems. The notion of directionality, such as up and down, is largely relative and dependent on the frame of reference.
In the case of our universe, it is commonly understood to be three-dimensional, without a well-defined "up" or "down." Our perspective of up and down is typically based on the gravitational pull of larger objects like planets. But on larger scales, such as the universe as a whole, there is no universally agreed-upon orientation.
Therefore, there isn't a definitive "above" or "underneath" the solar system or galaxies in the cosmic sense. The structure and arrangement of objects in the universe are complex and can span vast distances in various directions, but they don't adhere to a specific orientation as we might think of it locally within our planetary or galactic systems.