If we created a Terrain using any method, there we get the option to append the terrain (Yes/No). So, my doubt is, this step & Create Complex Terrain are 2 different things or its kind of similar thing ??? Basically, How this Two command are different

The Create Complex Terrain Model tool is used to merge or append multiple terrain models into one.

Create From Elements is used to create a terrain model from 3D graphical elements that is ruled.  Ruled Terrains update dynamically if the source elements are changed. Each element or group of elements of a terrain feature type can be selected individually or via selection set. This tool is useful if the elements do not have consistency symbology or attributes (where the Create From Graphical Filter Group tool may be a better option). After the terrain model is created from the first grouping of elements, the software prompts to append another terrain feature (by defining another selection set of elements) or to create another terrain model.

Once a Complex Terrain is created, can the underlying terrains be deleted, or must the rules on the Complex Terrain be disabled first?  I often merge a survey terrain with a larger, LiDAR-based  (or another method) terrain.  This becomes my target terrain when modeling.  At that point, the underlying terrains seem unnecessary.

Once a Complex Terrain is created, can the underlying terrains be deleted, or must the rules on the Complex Terrain be disabled first?  I often merge a survey terrain with a larger, LiDAR-based  (or another method) terrain.  This becomes my target terrain when modeling.  At that point, the underlying terrains seem unnecessary.