New Template inside Fixer Theme

I would like to to some integration for another plugin we use. This requires making a copy of the page template and replacing the WP loop with other code. I'm having trouble locating the php file with:

<?php if ( have_posts() ) :

Can you assist me with this?

EDIT: I just read can also use hooks....

By inserting a few lines in your theme’s functions.php file, First unhook the xplugin wrappers;

remove_action( 'xplugin_before_main_content', 'xplugin_output_content_wrapper', 10);
remove_action( 'xplugin_after_main_content', 'xplugin_output_content_wrapper_end', 10);

Then hook in your own functions to display the wrappers your theme requires;

add_action('xplugin_before_main_content', 'my_theme_wrapper_start', 10);
add_action('xplugin_after_main_content', 'my_theme_wrapper_end', 10);

function my_theme_wrapper_start() {
  echo '<section id="main">';
}

function my_theme_wrapper_end() {
  echo '</section>';
}