CSP, STS, X-Frame-Options, X-Content-Type, Referrer-Policy and Permissions-Policy

In case you can’t add in your Web Server, then you can use as PHP.
For the CSP, you can generate using the Chrome extension: CSP Generator.

<?php

add_action('send_headers', function () {

    header("Content-Security-Policy: default-src 'self'; script-src 'self' 'unsafe-inline' 'unsafe-eval' https://www.googletagmanager.com https://pagead2.googlesyndication.com; style-src 'self' 'unsafe-inline' https:; img-src 'self' data: https:; font-src 'self' data: https:; connect-src 'self' https://www.google-analytics.com; frame-src 'self' https://googleads.g.doubleclick.net; object-src 'none'; base-uri 'self'; form-action 'self';");
    header('Strict-Transport-Security: max-age=31536000; includeSubDomains');
    header('X-Frame-Options: SAMEORIGIN');
    header('X-Content-Type-Options: nosniff');
    header('Referrer-Policy: strict-origin-when-cross-origin');
    header('Permissions-Policy: geolocation=(), microphone=(), camera=()');

});