Thank you @Mark_Harwood
The hypothesis I can build out of your response is :
When 1000 fields are indexed, with the help of copy_to, these fields' values are indexed into one single field in Lucene.
Lucene would now contain only two fields i.e.,
_source which contains my JSON in its original format and
all_my_string_fields_indexed_as_one which is searchable.
Now on a Free text search, only
all_my_string_fields_indexed_as_one would be referred to. Hence, my free text would practically be on a single field
In case copy_to is not used, I would have 1001 fields in Lucene,
_source which contains my original JSON and 1000 fields which I have indexed that are searchable. So now on Free text search, all these 1000 fields are searched upon.
This is what I can apprehend. Am I missing something?