$( document ).ready(function() {

        // downloads page - taxonomy filter
        $('#edit-field-product-tree').change(function() {
          var selected_op = $(this).find('option:selected').text();
          if (selected_op.indexOf("-") >= 0) {
            /*
            var this_select_content = '<select id="states">';
            for(var i = 0; i < 10; i++){
              this_select_content += '<option value="' + i + '">' + i + '</option>';
            }
            this_select_content +='</select>'
            $("div.form-item-field-product-tree").append(this_select_content);
            */
            var s = $("<select id=\"selectId\" class=\"form-select\" name=\"selectName\" />");
            for(var i = 0; i < 10; i++){
              $("<option />", {value: i, text: i}).appendTo(s);
            }
            s.appendTo($(this).parent());

            console.log(selected_op);

          }

        });

      });




###
<div class="js-form-item form-item js-form-type-select form-type-select js-form-item-field-product-tree form-item-field-product-tree form-no-label">
<!-- THEME DEBUG -->
<!-- THEME HOOK: 'select' -->
<!-- BEGIN OUTPUT from 'core/themes/classy/templates/form/select.html.twig' -->
<select class="form-select" name="field_product_tree" id="edit-field-product-tree" data-drupal-selector="edit-field-product-tree"><option selected="selected" value="All">- Any -</option><option value="151">Others</option><option value="141">Selcom</option><option value="131">Primos</option><option value="136">Dynavision</option><option value="126">KScan3D</option><option value="146">HexSight</option><option value="11">Gocator</option><option value="3386">-Gocator G2</option><option value="3381">--Gocator 2300</option><option value="3391">---Gocator 2320</option><option value="16">chroma+scan</option><option value="6">HDI</option><option value="21">MikroCAD</option></select>
<!-- END OUTPUT from 'core/themes/classy/templates/form/select.html.twig' -->
</div>


##### RESULT ###

<div class="js-form-item form-item js-form-type-select form-type-select js-form-item-field-product-tree form-item-field-product-tree form-no-label">


<!-- THEME DEBUG -->
<!-- THEME HOOK: 'select' -->
<!-- BEGIN OUTPUT from 'core/themes/classy/templates/form/select.html.twig' -->
<select class="form-select" name="field_product_tree" id="edit-field-product-tree" data-drupal-selector="edit-field-product-tree"><option selected="selected" value="All">- Any -</option><option value="151">Others</option><option value="141">Selcom</option><option value="131">Primos</option><option value="136">Dynavision</option><option value="126">KScan3D</option><option value="146">HexSight</option><option value="11">Gocator</option><option value="3386">-Gocator G2</option><option value="3381">--Gocator 2300</option><option value="3391">---Gocator 2320</option><option value="16">chroma+scan</option><option value="6">HDI</option><option value="21">MikroCAD</option></select>
<!-- END OUTPUT from 'core/themes/classy/templates/form/select.html.twig' -->


        <select name="selectName" class="form-select" id="selectId"><option value="0">0</option><option value="1">1</option><option value="2">2</option><option value="3">3</option><option value="4">4</option><option value="5">5</option><option value="6">6</option><option value="7">7</option><option value="8">8</option><option value="9">9</option></select><select name="selectName" class="form-select" id="selectId"><option value="0">0</option><option value="1">1</option><option value="2">2</option><option value="3">3</option><option value="4">4</option><option value="5">5</option><option value="6">6</option><option value="7">7</option><option value="8">8</option><option value="9">9</option></select><select name="selectName" class="form-select" id="selectId"><option value="0">0</option><option value="1">1</option><option value="2">2</option><option value="3">3</option><option value="4">4</option><option value="5">5</option><option value="6">6</option><option value="7">7</option><option value="8">8</option><option value="9">9</option></select><select name="selectName" class="form-select" id="selectId"><option value="0">0</option><option value="1">1</option><option value="2">2</option><option value="3">3</option><option value="4">4</option><option value="5">5</option><option value="6">6</option><option value="7">7</option><option value="8">8</option><option value="9">9</option></select><select name="selectName" class="form-select" id="selectId"><option value="0">0</option><option value="1">1</option><option value="2">2</option><option value="3">3</option><option value="4">4</option><option value="5">5</option><option value="6">6</option><option value="7">7</option><option value="8">8</option><option value="9">9</option></select><select name="selectName" class="form-select" id="selectId"><option value="0">0</option><option value="1">1</option><option value="2">2</option><option value="3">3</option><option value="4">4</option><option value="5">5</option><option value="6">6</option><option value="7">7</option><option value="8">8</option><option value="9">9</option></select><select name="selectName" class="form-select" id="selectId"><option value="0">0</option><option value="1">1</option><option value="2">2</option><option value="3">3</option><option value="4">4</option><option value="5">5</option><option value="6">6</option><option value="7">7</option><option value="8">8</option><option value="9">9</option></select></div>