Need help writing a wp plugin. How to properly integrate external pages in wp?

Question

WordPress

The question has already been . The bottom line is that it was required to integrate a product catalog with its own database with the wp engine with wp. I'll tell you briefly, there is a database with goods that are replenished / deleted daily. single item. Two tables of the store base are used, this is a table with goods and a table with categories. The option of combining the wp base and the store base into one has disappeared. The easiest way is to inject your code with product display, categories into the wp theme without touching the wp code too much. In fact, these are "three" files:
product - displays product data
v_cat - displays products of a certain category
search - displays products by the word in the search
Actually everything is implemented crookedly, as far as knowledge and time was enough, namely:
three files product, v cat and search are located in the root of the site, at the very beginning they contained:

the code

<?php
require( dirname( __FILE__ ) . '/wp-blog-header.php' ); //загрузки фронтенда
 
include(TEMPLATEPATH."/product.php"); //подключение файла в теме с кодом страницы
?>

As you can see, the file is connected in the site template, in which the actual request and output code.
After that, in Yandex and Google, I began to notice that the pages already loaded in the search become 404. wp does not recognize these pages and sends the 404 code, as I understand it. After digging in Google, I found the same problem with a person on a foreign resource. After that, the content of the files in the root of the site changed, it became like this:

spoiler

<?php
require_once(dirname( __FILE__ ) . '/wp-config.php');
$wp->init(); 
$wp->parse_request(); 
$wp->query_posts();
$wp->register_globals(); 
$wp->send_headers();
//
include(TEMPLATEPATH."/product.php");
?>

a header was added for them, like this header-(product/category/search).
In general, it turns out a lot of unnecessary things and I want to put it all into a single plugin for convenience when updating wp and the template and so that everything is in one place. + for editing some product data in the wp admin panel.
Hence the question. How to properly build the plugin structure so that there is no superfluous. How are these 3 files that are in the root and make it clear to wp that these are its pages to be correctly implemented in the plugin?

Answer 1

Actually dug up Google and decided to do everything through the generation of virtual pages. I think it will be more convenient.