The meta tag should get inserted in the <head> section of a website. Regardless if you are using a header.php and footer.php or not, you should have a <head> section in your document.
For example the code in your document should be something like:
<!DOCTYPE html>
<html <?php language_attributes(); ?>>
<head>
<meta charset="<?php bloginfo( 'charset' ); ?>">
<meta name="viewport" content="width=device-width, initial-scale=1">
Also make sure you are using <?php wp_head(); ?> (before closing </head> section) and <?php wp_footer(); ?> (before closing </body> section) in your theme, as nearly all plugins depend on these. As this codes are default in the header.php and footer.php files.
I guess that is no real WP question it just seems about the meta tag and section.