They were all formed at the same time, from the same materials. We have sent spacecraft to all the terrestrial planets and landed on all but Mercury. The spacecraft have obtained higher resolution images from orbit around the planets, in infrared, ultraviolet and visual light, as well as spectroscopic data. Also radar maps, showing mountains and valleys, have been made of all the inner planets.
Some meteors are fragments of other planets, primarily Martian meteors.
From what we observe we can piece together the geologic history of the planets. The surface is a reflection of what's going on inside the planet. The rocky planets are spherical because of gravity. Gravity also is responsible for the layered structure of planet interiors. This is called differentiation.