Not going into where it came from. I already covered my position on something from nothing.
As to it's turning into everything, that's reasonably well understood.
Starting with A LOT of hydrogen, scattered widely in space, it is drawn together under the influence of gravity. The closer it gets together, the more gravity affects it. Eventually, it gets tightly packed together in huge balls of hydrogen that, at their cores, experience enough heat and pressure to drive a hydrogen fusion reaction. This is a star. Look up, you may see many of them.
Hydrogen fusion produces newer more complex elements like helium, but as the hydrogen begins to run out it will also undergo other forms of fusion, producing heavier elements up to the scale of iron. Some of these stars, if they are large enough will explode violently around the point where they transition from hydrogen to heavier fusion reactions, in an event called a supernova. Several such events have been observed by astronomers and the remains of many others are quite visible.
Supernova explosions are the source of most of the heavier elements above iron. They also tend to scatter a lot of matter around.
Subsequently, all this mess tends to converge back together under the force of gravity to form new or secondary star systems. Now with heavier elements around, they also form planets.
Most of the elements that make up out bodies, apart from the hydrogen in the water, was fused in a supernova.
This is why Carl Sagan famously said "We are all made of star stuff".
Star systems in all stages of this process can be observed with relatively straightforward astronomical equipment. We can tell which elements are present in a star my the telltale spectroscopic signature of each element.
Is that enough for you?
I'm typing this on my phone.