How to create a product attribute and assigned it all attribute set in magento 2

By default if you provide a value for 'group' in the attribute configuration array the attribute will be added to that group in all attribute sets:

$eavSetup->addAttribute('catalog_product', 'my_attribute', [
    ...
    'group' => 'General',  // or some other group name
    ...
]);

This might be helpful:-

Create InstallData.php in your Setup folder and paste this code in your file

<?php

namespace [Vendor]\[Module]\Setup;

use Magento\Eav\Setup\EavSetup; 
use Magento\Eav\Setup\EavSetupFactory;
use Magento\Framework\Setup\InstallDataInterface;
use Magento\Framework\Setup\ModuleContextInterface;
use Magento\Framework\Setup\ModuleDataSetupInterface;

class InstallData implements InstallDataInterface
{

    private $eavSetupFactory;

    public function __construct(EavSetupFactory $eavSetupFactory)
    {
        $this->eavSetupFactory = $eavSetupFactory; 
    }

    public function install(ModuleDataSetupInterface $setup, ModuleContextInterface $context)
    {
        /** @var EavSetup $eavSetup */
        $eavSetup = $this->eavSetupFactory->create(['setup' => $setup]);

        /**
         * Add attributes to the eav/attribute
         */
        $eavSetup->removeAttribute(\Magento\Catalog\Model\Product::ENTITY,'background_image');
        $eavSetup->addAttribute(
            \Magento\Catalog\Model\Product::ENTITY,
            'background_image',/* Custom Attribute Code */
            [

                'type' => 'varchar',/* Data type in which formate your value save in database*/
                'backend' => '',
                'frontend' => '',
                'label' => 'Background Image', /* lablel of your attribute*/
                'input' => 'media_image',
                'class' => '',
                'source' => '',
                                /* Source of your select type custom attribute options*/
                'global' => \Magento\Eav\Model\Entity\Attribute\ScopedAttributeInterface::SCOPE_STORE,
                                    /*Scope of your attribute */
                'visible' => false,
                'required' => false,
                'user_defined' => false,
                'default' => '',
                'searchable' => false,
                'filterable' => false,
                'comparable' => false,
                'visible_on_front' => false,
                'used_in_product_listing' => false,
                'unique' => false
            ]
        );


        $entityTypeId = $eavSetup->getEntityTypeId(\Magento\Catalog\Model\Product::ENTITY);
        $attributeSetIds = $eavSetup->getAllAttributeSetIds($entityTypeId);
        foreach ($attributeSetIds as $attributeSetId) {
            /**
             * getAttributeGroupId($entityTypeId, $attributeSetId, "Group_Code");
             * 
             */
            $groupId = $eavSetup->getAttributeGroupId($entityTypeId, $attributeSetId, "image-management");
            $eavSetup->addAttributeToGroup(
                $entityTypeId,
                $attributeSetId,
                $groupId,
                'background_image',
                null
            );
        }
    }
}

Here is the script by which can you add attribute to all attribute set.
Step 1:-Create InstallData.php in your Setup folder and paste this code in your file.

namespace Ktpl\BrandManagement\Setup;

    use Magento\Eav\Setup\EavSetup;
    use Magento\Eav\Setup\EavSetupFactory;
    use Magento\Framework\Setup\InstallDataInterface;
    use Magento\Framework\Setup\ModuleContextInterface;
    use Magento\Framework\Setup\ModuleDataSetupInterface;

    class InstallData implements InstallDataInterface
    {
    /**
     * EAV setup factory
     *
     * @var EavSetupFactory
     */
    private $eavSetupFactory;

    /**
     * Init
     *
     * @param EavSetupFactory $eavSetupFactory
     */
    public function __construct(EavSetupFactory $eavSetupFactory)
    {
        $this->eavSetupFactory = $eavSetupFactory;
    }

    /**
     * {@inheritdoc}
     * @SuppressWarnings(PHPMD.ExcessiveMethodLength)
     */
     public function install(ModuleDataSetupInterface $setup,     ModuleContextInterface $context)
    {
     /** @var EavSetup $eavSetup */
     $eavSetup = $this->eavSetupFactory->create(['setup' => $setup]);

     /**
     * Add attributes to the eav/attribute
     */

    $eavSetup->addAttribute(
        \Magento\Catalog\Model\Product::ENTITY,
            'shop_by_brand',
            [
                'type' => 'varchar',
                'backend' => '',
                'frontend' => '',
                'label' => 'Brand',
                'input' => 'multiselect',
                'class' => '',
                'source' => '',
                'backend' => 'Magento\Eav\Model\Entity\Attribute\Backend\ArrayBackend',
                'global' => \Magento\Catalog\Model\ResourceModel\Eav\Attribute::SCOPE_GLOBAL,
                'visible' => true,
                'required' => false,
                'user_defined' => false,
                'default' => 0,
                'searchable' => false,
                'filterable' => true,
                'comparable' => false,
                'visible_on_front' => true,
                'used_in_product_listing' => true,
                'unique' => false,
                'apply_to' => ''
            ]
        );
     }
   }

Please let me know if you have any query about it. please remove extra space if require.