do_action( 'admin_menu', string $context )

add_action( 'admin_menu', function () {
	add_menu_page(
		'Field Notes',                 // page <title>
		'Field Notes',                 // menu label
		'manage_options',              // capability required
		'field-notes',                 // menu slug, becomes ?page=field-notes
		'wppaste_render_field_notes',  // renders the screen
		'dashicons-clipboard',
		25                             // position, below Comments
	);
} );

function wppaste_render_field_notes() {
	if ( ! current_user_can( 'manage_options' ) ) {
		wp_die( esc_html__( 'You are not allowed to view this page.' ) );
	}

	echo '<div class="wrap"><h1>Field Notes</h1>';
	echo '<p>Look at the left-hand menu: this item was added by the snippet.</p>';
	echo '</div>';
}

add_action( 'admin_menu', function () {
	// Settings > Field Notes. The parent slug is the parent's file name.
	add_submenu_page(
		'options-general.php',
		'Field Notes Settings',
		'Field Notes',
		'manage_options',
		'field-notes-settings',
		function () {
			echo '<div class="wrap"><h1>Field Notes Settings</h1>';
			echo '<p>Open Settings in the sidebar to find this page.</p></div>';
		}
	);
} );

// Something else registers a menu at the default priority of 10.
add_action( 'admin_menu', function () {
	add_menu_page( 'Noisy Plugin', 'Noisy Plugin', 'manage_options', 'noisy-plugin', '__return_null', 'dashicons-megaphone', 26 );
} );

// Priority 999 runs last, so the item is there to remove.
add_action( 'admin_menu', function () {
	remove_menu_page( 'noisy-plugin' );
}, 999 );

// Removing the menu does not block the URL, so gate the screen too.
add_action( 'admin_init', function () {
	if ( isset( $_GET['page'] ) && 'noisy-plugin' === $_GET['page'] ) {
		wp_die( esc_html__( 'That screen has been disabled.' ) );
	}
} );