Yes, it is true that Magento provides default mobile open source codes that can be used to develop mobile applications for eCommerce websites. These mobile codes are available in both iOS and Android versions, which can be downloaded from the Magento Marketplace.
If you want to customize the mobile app using Magento, you can explore the code to a certain extent. Magento provides an API (Application Programming Interface) that allows developers to access and modify the website's data and functionalities from the mobile app.
To create an eCommerce app using Magento, you can use the API to integrate the app with the website's backend. This will allow the app to access product information, user data, and other essential features of the eCommerce website.
However, customization beyond the API's capabilities may require additional development work and expertise.