If you need to assign your custom attribute in all attribute set programmatically then you can used below steps:

$objectManager = MagentoFrameworkAppObjectManager::getInstance();
 
$attributeCode = 'is_featured';
$attributeGroup = 'General';
 
$eavSetup = $objectManager->create(MagentoEavSetupEavSetup::class);
$config = $objectManager->get(MagentoCatalogModelConfig::class);
$attributeManagement = $objectManager->get(MagentoEavApiAttributeManagementInterface::class);
 
$entityTypeId = $eavSetup->getEntityTypeId(MagentoCatalogModelProduct::ENTITY);
$attributeSetIds = $eavSetup->getAllAttributeSetIds($entityTypeId);
foreach ($attributeSetIds as $attributeSetId) {
 if ($attributeSetId) {
 $group_id = $config->getAttributeGroupId($attributeSetId, $attributeGroup);
 if(empty($group_id)){ continue; }
 $attributeManagement->assign(
 'catalog_product',
 $attributeSetId,
 $group_id,
 $attributeCode,
 100
 );
 }
}

Here, we performed the task using objectManager. But You can perform it via module.