Magento – Store product export csv file in var/importexport folder

please override Download.php file from the vendor.

1. Magento\ImportExport\Controller\Adminhtml\Export\File\Download.php

add changes to in.

 public function execute()
    {
        if (empty($fileName = $this->getRequest()->getParam('filename'))) {
            throw new LocalizedException(__('Please provide export file
name'));
        }
        try {
            $path = 'export/' . $fileName;
            $directory =
$this->filesystem->getDirectoryRead(DirectoryList::VAR_DIR);
            if ($directory->isFile($path)) {
                return $this->fileFactory->create(
                    *$fileName,*
                    $directory->readFile($path),
                    DirectoryList::VAR_DIR,
                    'application/octet-stream'
                );
            }
        } catch (LocalizedException | \Exception $exception) {
            throw new LocalizedException(__('There are no export file with
such name %1', $fileName));
        }
    }

Hopefully, it works for you.

Below Is a Default Class it only Export that Data which are show in your Admin Grid not Export your whole data

<exportButton class="Magento\Ui\Component\ExportButton">

If you want all Data you have create a Custom Export Button that export your all data.

edit in your Ui-Component file where Admin Grid Show

[vendor_name] \ [module_name] \view\adminhtml\ui_component

Add this code to add Button into Admin Grid

Ui-ComponentFIleName.xml

<item name="buttons" xsi:type="array">

    <item name="import" xsi:type="array">
        <item name="name" xsi:type="string">import</item>
        <item name="label" xsi:type="string" translate="true">Import</item>
        <item name="class" xsi:type="string">secondary</item>
        <item name="url" xsi:type="string">*/*/importdata</item>
        <item name="sortOrder" xsi:type="number">20</item>
    </item>

    <item name="export" xsi:type="array">
        <item name="name" xsi:type="string">export</item>
        <item name="label" xsi:type="string" translate="true">Export</item>
        <item name="class" xsi:type="string">secondary</item>
        <item name="url" xsi:type="string">*/*/exportdata</item>
        <item name="sortOrder" xsi:type="number">30</item>
    </item>

</item>

Now create a File that Export or Import Data. File Name must be same as you define in Ui-Component file

  <item name="url" xsi:type="string">*/*/exportdata</item>

  <item name="url" xsi:type="string">*/*/importdata</item>

File path must be like this

[vendor_name] \ [module_name] \ Controller \ Adminhtml \ [Controller_name] \ Exportdata.php

Exportdata.php

<?php

namespace [vendor_name]\[module_name]\Controller\Adminhtml\[Controller_name];

use Magento\Framework\App\Filesystem\DirectoryList;


class Exportdata extends \Magento\Backend\App\Action
{
    protected $uploaderFactory;

    protected $_locationFactory; 

    public function __construct(
        \Magento\Backend\App\Action\Context $context,
        \Magento\Framework\App\Response\Http\FileFactory $fileFactory,
        \Magento\Framework\Filesystem $filesystem,
        \[vendor_name]\[module_name]\Model\locatorFactory $locationFactory // This is returns Collaction of Data

    ) {
       parent::__construct($context);
       $this->_fileFactory = $fileFactory;
       $this->_locationFactory = $locationFactory;
       $this->directory = $filesystem->getDirectoryWrite(DirectoryList::VAR_DIR); // VAR Directory Path
       parent::__construct($context);
    }

    public function execute()
    {   
        $name = date('m-d-Y-H-i-s');
        $filepath = 'export/export-data-' .$name. '.csv'; // at Directory path Create a Folder Export and FIle
        $this->directory->create('export');

        $stream = $this->directory->openFile($filepath, 'w+');
        $stream->lock();

        //column name dispay in your CSV 

        $columns = ['Col-1-name','Col-2-name','Col-3-name','Col-4-name','Col-5-name','Col-6-name','Col-7-name','Col-8-name','Col-9-name',];

            foreach ($columns as $column) 
            {
                $header[] = $column; //storecolumn in Header array
            }

        $stream->writeCsv($header);

        $location = $this->_locationFactory->create();
        $location_collection = $location->getCollection(); // get Collection of Table data 

        foreach($location_collection as $item){

            $itemData = [];

            // column name must same as in your Database Table 

            $itemData[] = $item->getData('col-1-name');
            $itemData[] = $item->getData('col-2-name');
            $itemData[] = $item->getData('col-3-name');
            $itemData[] = $item->getData('col-4-name');
            $itemData[] = $item->getData('col-5-name');
            $itemData[] = $item->getData('col-6-name');
            $itemData[] = $item->getData('col-7-name');
            $itemData[] = $item->getData('col-8-name');
            $itemData[] = $item->getData('col-9-name');

            $stream->writeCsv($itemData);

        }

        $content = [];
        $content['type'] = 'filename'; // must keep filename
        $content['value'] = $filepath;
        $content['rm'] = '1'; //remove csv from var folder

        $csvfilename = 'locator-import-'.$name.'.csv';
        return $this->_fileFactory->create($csvfilename, $content, DirectoryList::VAR_DIR);

    }


}

Now you can Click on the Export Button and See your .csv file Downloaded below.

I Hope This Helps You.