Premade Assets are all over the internet and in a lot of cases freely available. If you don’t have skills in art or sound design, no problem. Go onto the Unity Asset Store and check out either free or purchasable assets that are ready to use for your game!
Any decent quality game or software of any sort is going to need a multitude of various assets in order to ship. This means Scripts, 2D Sprites, 3D Models, Textures, Animations, Audio Samples, the list goes on. Nobody is perfect, and nobody is capable of producing AAA quality assets for every aspect of a game.