How getEntities() function is getting values from in magento2 import export module

I am working on creating a custom extension for import export in magento2. For that I was going through magento2 import-export module. I found a function in file

/vendor/magento/module-import-export/Model/Import.php

protected function _getEntityAdapter()
{
    if (!$this->_entityAdapter) {
        $entities = $this->_importConfig->getEntities();
        if (isset($entities[$this->getEntity()])) {
            try {
                $this->_entityAdapter = $this->_entityFactory->create($entities[$this->getEntity()]['model']);
            } catch (Exception $e) {
                $this->_logger->critical($e);
                throw new LocalizedException(
                    __('Please enter a correct entity model.')
                );
            }
            if (!$this->_entityAdapter instanceof AbstractEntity &&
                !$this->_entityAdapter instanceof ImportAbstractEntity
            ) {
                throw new LocalizedException(
                    __(
                        'The entity adapter object must be an instance of %1 or %2.',
                        AbstractEntity::class,
                        ImportAbstractEntity::class
                    )
                );
            }

            // check for entity codes integrity
            if ($this->getEntity() != $this->_entityAdapter->getEntityTypeCode()) {
                throw new LocalizedException(
                    __('The input entity code is not equal to entity adapter code.')
                );
            }
        } else {
            throw new LocalizedException(__('Please enter a correct entity.'));
        }
        $this->_entityAdapter->setParameters($this->getData());
    }
    return $this->_entityAdapter;
}

In the second line of the function ie

$entities = $this->_importConfig->getEntities();

By reading it I understood getEntities function() is fetching entities that are meant to be imported like products, customers.
I want to know where these values like products and customers are created so that magento treats them as importable or exportable entities.
To find out this I tried to locate _importConfig in the above line.
It was passed in construct like

use MagentoImportExportModelImportConfigInterface;
public function __construct(
    LoggerInterface $logger,
    Filesystem $filesystem,
    DataHelper $importExportData,
    ScopeConfigInterface $coreConfig,
    ConfigInterface $importConfig,
    Factory $entityFactory,
    Data $importData,
    CsvFactory $csvFactory,
    FileTransferFactory $httpFactory,
    UploaderFactory $uploaderFactory,
    BehaviorFactory $behaviorFactory,
    IndexerRegistry $indexerRegistry,
    History $importHistoryModel,
    DateTime $localeDate,
    array $data = [],
    ManagerInterface $messageManager = null,
    Random $random = null
) {
    $this->_importExportData = $importExportData;
    $this->_coreConfig = $coreConfig;
    $this->_importConfig = $importConfig;
    $this->_entityFactory = $entityFactory;
    $this->_importData = $importData;
    $this->_csvFactory = $csvFactory;
    $this->_httpFactory = $httpFactory;
    $this->_uploaderFactory = $uploaderFactory;
    $this->indexerRegistry = $indexerRegistry;
    $this->_behaviorFactory = $behaviorFactory;
    $this->_filesystem = $filesystem;
    $this->importHistoryModel = $importHistoryModel;
    $this->localeDate = $localeDate;
    $this->messageManager = $messageManager ?: ObjectManager::getInstance()
        ->get(ManagerInterface::class);
    $this->random = $random ?: ObjectManager::getInstance()
        ->get(Random::class);
    parent::__construct($logger, $filesystem, $data);
}

Now in the file
/vendor/magento/module-import-export/Model/Import/ConfigInterface.php
I found the function

/**
 * Retrieve import entities configuration
 *
 * @return array
 */
public function getEntities();

But this function was empty, it may be because of interface. I need to read more to understand about interface what it actually does. But for now I found where this interface was used and in this file I think it might be

/vendor/magento/module-import-export/Model/Import/Config.php

class Config extends MagentoFrameworkConfigData implements MagentoImportExportModelImportConfigInterface
{
    public function getEntities()
    {
        return $this->get('entities');
    }
}

Here also I could not locate where entities are defined. I looked for get function in the above class parent class
/vendor/magento/framework/Config/Data.php

/**
 * Get config value by key
 *
 * @param string $path
 * @param mixed $default
 * @return array|mixed|null
 */
public function get($path = null, $default = null)
{
    if ($path === null) {
        return $this->_data;
    }
    $keys = explode('/', $path);
    $data = $this->_data;
    foreach ($keys as $key) {
        if (is_array($data) && array_key_exists($key, $data)) {
            $data = $data[$key];
        } else {
            return $default;
        }
    }
    return $data;
}

In this get function comments it was written get config value by key. By that I understood these config values are defined somewhere and entities is the key.
I am having problem in finding how and where these entities are defined and how I can make a custom entity to import.