Theme Integration to Other Files?

Hello there! I’m new to PHP and WordPress, but yes, I’m already enjoying it. I have a question though if there’s a possible way I can integrate my WordPress themes to other PHP files? Perhaps a little code to include as header and footer? I’m using a theme switcher.

And one more thing, I’m also looking for the template tag that will list and link only the parent pages without the child pages, sadly, I couldn’t find any.

Thank you very much. Any help will be greatly appreciated. ??