Insert custom content after second paragraph of content

functions.php

<?php

declare( strict_types=1 );

namespace Blockify\Test;

use function add_filter;
use function explode;
use function implode;
use function is_singular;

add_filter( 'the_content', __NAMESPACE__ . '\\insert_after_second_paragraph' );
/**
 * Insert custom content after second paragraph of post content.
 *
 * @param string $content The content.
 *
 * @return string
 */
function insert_after_second_paragraph( string $content ): string {
	if ( ! is_singular( [ 'post', 'page' ] ) ) {
		return $content;
	}

	// Change this to the paragraph number you want to insert the content after.
	$count = 2;

	// Change this to your own content.
	$custom_content = '<br/><strong>Custom Content</strong>';

	$explode           = explode( "</p>", $content );
	$explode[ $count ] = $custom_content . $explode[ $count ];

	return implode( '</p>', $explode );
}