Hello, first thanks for your purchase.
The best way is use a server side code language such as PHP to create header.php and footer.php files. Then for each new page you create you can include these partials.
Learn more about that here:
https://stackoverflow.com/a/35769918
We do not recommend use jQuery.load(), since this is very bad for user experience and SEO (google search). Also you will have problems like sticky header not working. This occurs because the code for sticky header is being executed before the header html content it’s in the page. If you want to proceed with it, you need make sure the HTML of header it’s already in the DOM before executing the theme core scripts.
We hope this helps!
Let us know if you need further assistance.
Kind Regards,
Rodrigo.
If you are satisfied with Porto HTML and our support, please leave your comment and rating on ThemeForest downloads page. That helps us a lot: https://themeforest.net/downloads