merge will merge object hierarchy from source to destination. While for extend/assign, it’s simple one level copy of properties from source to destination.
For each property in source, copy its value as-is to destination. if property values themselves are objects, there is no recursive traversal of their properties. Entire object would be taken from source and set in to destination.
For each property in source, check if that property is object itself. If it is then go down recursively and try to map child object properties from source to destination.