If you want to implement “add select all button in Export Product” functionality in Magento 2, then please follow the below step.

Step 1. Create Magemonkey/Exportproduct/registration.php file and add below code

<?php
MagentoFrameworkComponentComponentRegistrar::register(
    MagentoFrameworkComponentComponentRegistrar::MODULE,
    'Magemonkey_Exportproduct',
    __DIR__
);

Step 2. Create file of Magemonkey/Exportproduct/etc/module.xml

<?xml version="1.0" ?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Module/etc/module.xsd">
    <module name="Magemonkey_Exportproduct" setup_version="1.0.0">
        <sequence>
            <module name="Magento_Backend"/>
        </sequence>
    </module>
</config>

Step 3. Create file of Magemonkey/Exportproduct/etc/adminhtml/di.xml

<?xml version="1.0" ?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <type name="MagentoBackendBlockWidgetGridExtended">
        <plugin disabled="false" name="Magemonkey_Exportproduct_Plugin_Backend_Magento_Backend_Block_Widget_Grid_Extended" sortOrder="10" type="MagemonkeyExportproductPluginBackendMagentoBackendBlockWidgetGridExtended"/>
    </type>
</config>

Step 4. Create plugin file of Magemonkey/Exportproduct/Plugin/Backend/Magento/Backend/Block/Widget/Grid/Extended.php

<?php
namespace MagemonkeyExportproductPluginBackendMagentoBackendBlockWidgetGrid;
class Extended
{
    protected $request;

	public function __construct(
        MagentoFrameworkAppRequestHttp $request
    ){
        $this->request = $request;
    }
    public function afterGetMainButtonsHtml(
        MagentoBackendBlockWidgetGridExtended $subject,
        $result
    ) {
    	$moduleName = $this->request->getModuleName();
        $controller = $this->request->getControllerName();
        $action     = $this->request->getActionName();
        $route      = $this->request->getRouteName();

        if($controller == 'export' && $action == 'getFilter'){ //check controller and action
        $result .= '<button id="" title="Select All" type="button" class="action-default scalable action-reset action-tertiary" onclick="export_filter_gridJsObject.selectAll()" data-action="grid-filter-reset" data-ui-id="widget-button-3"><span>Select All</span></button>';
    	}else{
    	$result .= '';
    	}
        return $result;
    }
}

Step 5. Override the /vendor/magento/module-import-export/view/adminhtml/templates/export/form/filter/after.phtml file and add below code after line 18

export_filter_gridJsObject.selectAll = function () {
            // class admin__control-checkbox is selector 
            var inputs = document.getElementsByClassName("admin__control-checkbox");
            for(var i = 0; i < inputs.length; i++) {
                if(inputs[i].type == "checkbox") {
                    inputs[i].checked = true;
                }
            }
        };

Note. Create the admin theme using this link and follow that step https://magemonkeys.com/magento-2-change-default-logo-of-admin-panel