RankMath and other WordPress plugins tend to inject some HTML into the content of the page. This code is not normally seen by the end users but when somebody views the page’s source code they will see it. It’s extra 200-300 characters.
Of course if you’ve worked hard on a plugin you’d want the sites that are using it to show it. We have this approach at our parent company Orbisius too.
If you want for some reason to not include the branding in the HTML code you can add this code to the functions.php of your (child) theme or a custom plugin of yours.
add_filter('rank_math/frontend/remove_credit_notice', '__return_true', 20);