1) the results page is set to filter by category
edit the database search behavior on the results page to add a Categoty filter using the CategoryID url variable.
2) the databASe search behavior on the results page is set to trigger on a URL variable named search
your category links:
<a href="../../products_results.php?CategoryID=<?php echo $row_menuwriter_productcategories["CategoryID"]; ?>"><?php echo $row_menuwriter_productcategories["CategoryName"]; ?></a>
will need to contain the search URL variable:
<a href="../../products_results.php?CategoryID=<?php echo $row_menuwriter_productcategories["CategoryID"]; ?>&Search=Search"><?php echo $row_menuwriter_productcategories["CategoryName"]; ?></a>