Opencart Modification System Related Questions

I have been using opencart since a short time. But I know nothing about Vqmod, Ocmod and third part extensions. I have few questions about them. I have searched Goolge and forum but did not find a solution, so I came to you for help. I would be greately thankful if someone guides me.

(1) Does Ocmod or Vqmod installation makes any changes to core files of oencart

(2) Does Ocmod or Vqmod installation is necessary to avoid changes in core files, if we install anyother third party extensions

(3) If we install a third party extension without ocmod.zip or vqmod.zip file extension manually, does it make changes to core files

(4)Has Ocmod and Vqmod been developed by opencart itself or it is developed by independent developers and opencart community.

(5) Does different ocmods or vqmods are needed to make changes in different files for example theme, controller and catalog or same ocmod or vqmod can perform all the tasks

(6) Where to download standard ocmod or vqmod to make changes in opencart without efecting the core files. I went to opencart market place. I found ocmod manager, ocmod editor, ocmod downloader. But I did not find ocmod or vqmod extension
to make changes in opencart files for example theme without effecting core files.