Sculptamold, Hydrocal, Plaster of Paris?

Can someone give a brief explanation as to when you would use these? I am about to embark on my first layout and I need some advice. From what I can tell, sculptamold is used mostly for terrain

Scenery basics for model railroaders

Sculptamold : Model Making : Modeling And Mold Making

Should I Use Sculptamold or Something Else? - page 1 - Scenery Techniques - TheRailwire

Hydrocal Plaster for Scenery, Dioramas, Dentistry and Mold Casting 5 lb Pack Resealable Bag Great for Model Railroads by Capital Ceramics : Arts, Crafts & Sewing

Hydrocal White Available in the US and Canada - Reynolds Advanced Materials

Plaster of Paris or Hydrocal, Page 2

Tips for rock texture? : r/TerrainBuilding

Using Sculptamold for the First Time: Need Some Useful Tips

Lightweight Hydrocal versus Sculptamold

Hydrocal Plaster - Regular Set - Scenery Plaster - Model Railroad - 5 lb Bag - Direct from Manufacturer