which .php file is the one I want for changing the header?

I have been casting around trying to find the file I need to edit in order to change the header in my Holder-Block home page. Both the index.php and the page.php have a get_header () command, and I would like to create a new header .php file to be called, but I want to use the existing header file as a template that I will edit. How do I find this file? I have found a header.php file under the top holder folder, a block-header.php file under the library/components/format/block folder, and an options-header.php file under the library/components folder. Which of these is the file I want to use as the template for my new header file? Or should I be looking for some other file that is called in one of these header.php files?
Thanks in advance.