How do planets solar systems and the universe differ?

Answer 1

The inner planets, Mercury, Venus, Earth, and Mars, are mostly rock and iron. The outer planets, Jupiter, Saturn, Uranus, and Neptune, are much larger and are mostly hydrogen, helium, and ice.

Answer 2

How does the universe differ from what? We only know of the one, so it's hard to imagine what we could compare it to.

If you mean how do these things differ from each other... pretty much completely.

A planet is a celestial body that isn't a star, is large enough to have its own weight pull it into hydrostatic equilibrium (you can basically think of this as "being round"), and has "cleared its orbit", which is a little complicated but basically means that it's the biggest thing anywhere near its own orbit by a large margin (some exceptions could be made for "double planets", where two similarly-sized objects share the same orbit and also orbit each other).

A solar system consists of at least one star and at least one other object, which could be another star or a planet or even just gravitationally bound debris (like asteroids or comets).

The universe is everything. Which is why we only know of the one, there's only one set of "everything". It's possible to quibble about this and talk about parallel universes or alternate universes which are separate from ours in some way, but we don't know of any and probably can't know of any.