Technically you can get away with only adding a snippet to functions.php if you want that defines your main content wrapper:

https://woocommerce.com/document/woocommerce-theme-developer-handbook/

remove_action( 'woocommerce_before_main_content', 'woocommerce_output_content_wrapper', 10);
remove_action( 'woocommerce_after_main_content', 'woocommerce_output_content_wrapper_end', 10);

Then:

add_action('woocommerce_before_main_content', 'my_theme_wrapper_start', 10);
add_action('woocommerce_after_main_content', 'my_theme_wrapper_end', 10);

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

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

What are the exact PHP theme files that WooCommerce uses?

Mastering the CSS

ah yes that is chapter 3 in Mastering los Interwebz