Site pages show the page title under navigation bar

Your theme (or child theme) is likely using page.php as the default page template. This file likely contains a call to the WP function the_title() which is generating the visible page title.

Multiple options to modify this. Assuming you are using a theme which will have updates available, your changes should be made using the Child Theme approach in WordPress. https://codex.wordpress.org/Child_Themes. This will ensure your changes survive an update to the parent theme.

Assuming you never want the page title to show, simply removing the call to the_title() and any associated markup (heading tags, etc) will eliminate this.