WordPress "Hello World!" plug-in adds itself to all new pages?

I created my first PHP WordPress plugin and find that it ads itself to any new page, but I want to be able to add it when I want to.

I was not sure how much of a header, or any header, may be required by WP to be considered a plug-in.

I zipped it up, along with an empty index.html file, then zipped it into hello.zip. I added the plugin using the WP Admin panel, and found it worked.

BUT, any new page gets "Hello World!"

How do I prevent it from going into every page? And how do I place the plug-in on a page, where I would like it to appear?

 <?php
/**
 * Hello World! for WordPress
 *
 * @package   hello
 * @link      https://patriotssoar.com
 * @author    My name
 * @copyright 2021 My name
 *  *
 * Plugin Name:  hello
 * Description:  A test WordPress plugin.
 * Version:      1.0
 * Plugin URI:   https://mydomain.com
 * Author:      My name
 * Author URI:   https://mydomain.com
 * Text Domain:  hello
 * Domain Path:  /languages/
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  
 */
    echo "Hello World!";
?>

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