The formation of the Solar system was a very dynamic process. A lot of matter was thrown towards the outer solar system which further formed the Gas giants: Jupiter, Saturn, Uranus, and Neptune. The size of these outer planets is huge so is their gravity.
Because of their huge gravity a lot of matter which was scattered in the outer solar system got attracted towards them. This matter is what make the rings of the outer planets. Also, because of immense gravity they captured larger bodies thus making them their Moons.
