Stamping features on the terrain is a process which is supported by Unity out of the box. Simply put stamping is blending of various textures with each other. This process has been existing for a very long time and has been used in various applications, most notably image processing applications like Photoshop, Krita, Gimp, you name it.
Unity provides a very convenient means to blend those images together with their built-in terrain tools. The Unity terrain tools provide an editor with a brush preview which allows you to adjust a stamp in detail before you apply it.
For an even better experience the free Unity Terrain Tools package which can be installed in addition to the built-in tools via Package Manager is an excellent choice. It extends the terrain tools palette with even more tools which allow you to fine tune your stamped terrain to achieve the look of your personal artistic preference.
