I'm trying to publish newsletters online (I create all the newsletters using Newsletter module)

1. I added additional field called "Publish_Online" to the NewsletterType.php and Newsletter.php

2. I created new page types called NewsletterHolder and NewsletterPage.

3. In the NewsletterHolder I included a function:

public function NewsletterTypes() {
      return DataObject::get("NewsletterType","");
   }

to get all the data from the table NewsletterType from db.

In the NewsletterHolder template I have this code:

<% if NewsletterTypes %>   
      <% control NewsletterTypes %>
          <% if Publish_Online="true" %>
               <% if DraftNewsletters %>
                  <% control DraftNewsletters %>
                      <% if Publish_Online="true" %>
                        <p>$Title<br>Date: $Created<br><a href="$baseURL/newsletter/?id={$ID}">Read</a></p>
                      <% end_if %>
                  <% end_control %>
               <% end_if %>
               
               <% if SentNewsletters %>
                  <% control SentNewsletters %>
                      <% if Publish_Online="true" %>
                        <p>$Title<br>Date: $Created<br><a href="$baseURL/newsletter/?id={$ID}">Read</a></p>
                      <% end_if %>
                  <% end_control %>
               <% end_if %>
          <% end_if %>
      <% end_control %>
    <% end_if %>

4. Now when I click on the link <a href="$baseURL/newsletter/?id={$ID}">Read</a> a go to the page (with type "NewsletterPage").
And I need to read this parameter id={$ID}? How to do that?
Or there are other ways to pass parameters between pages?

I know that I'm talking to myself here.
But I discovered, that to pass parameters between page1 and page2 you need to define a function in the page1 controller,
for example like in the newsletter module:

public function preview($request) {
$newsletterID = (int) $request->param('ID');
$obj = DataObject::get_by_id('Newsletter', $newsletterID);
return $this->customise($obj)->renderWith("page2");
}

and than the link like "http://www.mysite.com/page1/preview/12" inside "http://www.mysite.com/page1/"
would go through that function "preview", and the parameter $request would be "12".
Using renderWith("page2") in the function "preview" you can define a new page based on "themes/mytheme/Layout/page2.ss".

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Please advise where to read about this process more? And did I understand everything correctly?

thanks