Default Magento does not show associate product’s id of a configurable product on the product detail page When change options of a configurable product.

If, you want to show associate id then see below code

Override this file into your theme : vendor/magento/module-catalog/view/frontend/templates/product/view/attribute.phtml

And, paste below script in last of this page

<script type="text/javascript">
    
    require(["jquery"], function(jQuery) {
        
        jQuery(".product-options-wrapper select[id^='attribute']").last().on('change', function() {
            
            setTimeout(function (){
                
                simpleId=jQuery("input[name=selected_configurable_option]").val();
                
                jQuery(".value").html(simpleId);       // Place your place id or class in place of ".value"

            }, 500); 

        });

    }); 

</script>