..but shows results from all Categories.
To fix this, add these few lines to shop_browse_queries.php, the ones that are between comments: new and end.

<?php
...
// Filter Products by Category
if( $category_id ) {
    // new: refine manufactures within current category
    if( !empty( $manufacturer_id ) ){            
        $where_clause[] = "`#__{vm}_product_category_xref`.`category_id`=".$category_id;
    }
    // end
    
    if( !empty( $search_this_category ) && (!empty( $keyword ) || !empty( $manufacturer_id ) )) {
        $where_clause[] = "`#__{vm}_product_category_xref`.`category_id`=".$category_id;
    } elseif( empty( $keyword ) && empty( $manufacturer_id )) {
        $where_clause[] = "`#__{vm}_product_category_xref`.`category_id`=".$category_id;
    }
}