Child Themes – Customization and Security Issues

Forgive me if this is a stupid question or posted in the wrong forum.

I’d like to know how to securely make modifications to some of the functionality in WordPress. Such as how to remove all mentions of the author.

I read about Child themes and how its possible to make a copy of the files like index.php and make mods to that so that the original index.php theme file remains intact during theme updates.

My question is, if I have an old index.php in my child theme folder won’t that be a security risk? Or would that version only contain the code for my mods and nothing else?

If the latter how do I add some code in there which I simply wanted to have removed? For example the ‘author’ field? Is there some method of adding in a couple of lines of code that you’d prefer to have excluded?

Sorry if this sounds utterly gormless.