Magento2: Add a simple product to a configurable product

magento2_add_simple_to_configurable_product.php

<?php

class ProductManager implements ProductManagerInterface
{
    /**
     * @var \Magento\Catalog\Api\ProductRepositoryInterface
     */
    private $prodRep;

    public function __construct()
    {
        $this->prodRep = $this->container->get(
            \Magento\Catalog\Api\ProductRepositoryInterface::class
        );
    }

    public function prodAssign() 
    {
        $skuConfigurable = '11111/';
        $skuSimple = '11111/222/XL';
        
        // Choose two products; one simple and one configurable
        $mySimpleProduct = $this->prodRep->get($skuSimple);
        $myConfigurableProduct = $this->prodRep->get($skuConfigurable);

        // The attributes "ColourCode" and "Size" must exist on the simple!
        echo "Color: " . $mySimpleProduct->getColourCode() . PHP_EOL;
        echo "Size: " . $mySimpleProduct->getSize() . PHP_EOL;

        // create arrays with all available colour codes and sizes (one of each only in this example)
        $confProd['size'] = [$mySimpleProduct->getSize()];
        $confProd['colour_code'] = [$mySimpleProduct->getColourCode()];

        // create arrays of optionValues
        $valsProd = [];
        $valsProd['size'] = [];
        foreach ($confProd['size'] as $size) {
            $myConfigurableOptionValue = $this->optionValueInterfaceFactory->create();
            $myConfigurableOptionValue->setValueIndex($size);
            $valsProd['size'][] = $myConfigurableOptionValue;
        }
        $valsProd['colour_code'] = [];
        foreach ($confProd['colour_code'] as $size) {
            $myConfigurableOptionValue = $this->optionValueInterfaceFactory->create();
            $myConfigurableOptionValue->setValueIndex($size);
            $valsProd['colour_code'][] = $myConfigurableOptionValue;
        }
        
        // create configurable options
        $configurableOption1 = $this->optionInterfaceFactory->create();
        $configurableOption1->setAttributeId($this->getAttributeIdFromCode('colour_code'));
        $configurableOption1->setValues($valsProd['colour_code']);
        $configurableOption1->setLabel('Colour');

        $configurableOption2 = $this->optionInterfaceFactory->create();
        $configurableOption2->setAttributeId($this->getAttributeIdFromCode('size'));
        $configurableOption2->setValues($valsProd['size']);
        $configurableOption2->setLabel('Size');
        
        // Assign the configurable options to the configurable product
        $myConfigurableProduct->getExtensionAttributes()->setConfigurableProductOptions(
            [
                $configurableOption1,
                $configurableOption2
            ]
            );
        $this->prodRep->save($myConfigurableProduct);

        // add the Simple to the Configurable
        $this->linkManagementInterface->addChild($skuSimple, $skuConfigurable);        
    }
}