Theres two ways I know of that you can go about making rocks without using plugins or blender. The first is using unions to shape a part into a desired rock formation and paint it a color and material thats makes it look like a rock (I get the best results painting it gray and making its material slate. You could also use the terrain tools to grow and shape rocks. Theres a section in this post on how to use terrain to create rocks here
The steps for the terrain rocks are under the “Terrain Sculpting” section