When you have different files; say different chapters of a book, that are written in separate Tex files, all having their own preamble packages and document class options, how does it work when you
\include them in one file with its own preamble options. Do they use the root file preamble alone for things like
a4paper,openleft options? Do different options and packages add up from different files?
I also understand that
\input uses the preamble of the root file alone, is this true? and if so, would that make it better than
\include; as in easier to manage?