Snow line
Why aren’t the inner planets gas giants?
Close to the Sun it was too warm for ice and light gas to stick. The inner four could only build from rock and metal, so they stayed small. Past a line called the snow line, ice piled on, gravity grabbed hydrogen and helium, and the giants grew huge. The solar system simulator still shows that split: four small inner worlds, then a jump.
A temperature line in the disc
In the disc around the young Sun, water was vapour close in and ice farther out. Rock could stick either side of that line. Ice could not stick inside it. So the inner four grew from rock and metal only, and never got massive enough to hold a thick envelope of hydrogen.
Outside the line, ice was extra building material. More mass, more gravity, and those worlds could pull in the leftover gas before the Sun blew it away. Jupiter and Saturn won that race. Uranus and Neptune got there later and grabbed less gas, which is why they are ice giants rather than hydrogen balls.
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The missing super-Earth
Planets between the size of Earth and Neptune are the commonest kind found around other stars, and our system does not have one. There is a gap between Earth and Neptune where most systems put their most typical planet. Jupiter’s early drift is the usual explanation. It is a model, not a photograph of the past.