I need a terrain generator

You can look into terrain generation software available on the market, or into the algorithms behind them by googling the topics, to learn how to write your own generation.

Unity does support "landscapes" and offers tools to manually sculpture landscapes, which you can also look into because this would later use your generated heightmap to generate the landscape from. Applying a texture onto this landscape, esp that it has snowy mountains, will have to be handled separately.

Having clouds at a particular y level is just a matter of placing your clouds at the correct height (given that you have clouds to place), but has generally nothing to do with the generation of the landscape.

But this is imo simply put "beyond the scope of a single question". So I can only tell you some pointers to where you can start looking for resources.