"How does the solar nebula theory explained ?

It all started with a cloud of gas.

Due to gravity the clouds will start to shrink. In the core density and temperature will become high enough so that nuclear fusion can start. The gas cloud becomes a stable star. This is the present state of our Sun.

Around the Sun a disk of gas and dust is left, containing about 1% of the mass of the Sun. In the inner part of the disk it is warm enough so that molecules like water, ammonia and methane tend to stay as gases and not produce grains and clumps and so on. And because they stay in gas form the radiation pressure from the Sun and the solar wind will push them outwards.

Around the orbit of Jupiter it becomes cold enough for ice to form. The gaseous molecules can produce grains and lumps so there's a lot of this less dense material around to accrete into planets. Jupiter and Saturn grew large enough to pull in great quantities of hydrogen and helium from the solar nebula.

The inner planets accreted from grains containing heavier atoms like oxygen and aluminium and silicon as well as iron and nickel and so on.

Consequently inside the ice limit at 5 AU only small, dense planets have formed, while outside there is matter enough to form the gas giants.

At this stage we have a dozen of proto-planets and quite a lot of comet-like, icy, small stuff. The proto-planets swept space clean with their gravitation fields and the comets rained on them. This also helped in making their orbits more circular.

But it was still a period with big collisions. For instance there are clear indications that the Moon was formed after a collision of the proto-Earth with another big object.

Nowadays, what is left over are zones like the asteroid belt, where Jupiter's perturbing gravity inhibited the formation of another planet. And also the Oort cloud, the region outside the orbit of Neptune, where dwarf planets like Pluto and Eris make their long orbits.