The archive page is shown when a category is selected and typically has excerpts of each post in that category

Category page hook

add_filter('the_content', 'our_modify_category_content');
function our_modify_category_content ($Content)
{
  
  //Exit if not category
  if (!is_category() || !is_main_query())
    return($Content);

  //$Category = get_queried_object();
  //$CategoryName = $Category->name;

  $Output = '';
  $Output .= '[Inserting at start]<br>';
  $Output .= $Content;
  $Output .= '[Inserting at end]';
  return $Output;
}

Doesn’t work?

Maybe your theme has overridden the default WordPress category pages with its own custom category page?

Theme hooks

Look in your theme’s archive.php (or category.php) file – does it have any hooks?

Kleo theme example hook functions

//****************************************************************
//****************************************************************
//********** WP CATEGORY ARCHIVE PAGE - BEFORE CONTENT ***********
//****************************************************************
//****************************************************************
add_action( 'kleo_before_archive_content', 'odac_kleo_before_archive_content' );     //kleo_before_archive_content is a hook kleo has in its archive.php
function odac_kleo_before_archive_content( )
{
  
  //-----------------------
  //----- HTML OUTPUT -----
  //-----------------------
  
  $HtmlOutput = "";
  
  //----- DISPLAY THE CATEGORY TITLE -----
  if (is_archive())
  {
    $current_category = single_cat_title("", false);
    
    $HtmlOutput .= "<h1>$current_category</h1>";
  }
  
  echo $HtmlOutput;
}


//***************************************************************
//***************************************************************
//********** WP CATEGORY ARCHIVE PAGE - AFTER CONTENT ***********
//***************************************************************
//***************************************************************
add_action( 'kleo_after_archive_content', 'odac_kleo_after_archive_content' );     //kleo_after_archive_content is a hook kleo has in its archive.php
function odac_kleo_after_archive_content( )
{
  
  //-----------------------
  //----- HTML OUTPUT -----
  //-----------------------
  
  $HtmlOutput = "";
  
  //----- DISPLAY THE HOME AND BACK BUTTONS -----
  if (is_archive())
  {
    $HtmlOutput .= "THE END";
  }
  
  echo $HtmlOutput;
}