Can we customize the Shopify default APIs? Like if we have a website on Shopify and want to integrate those APIs with the mobile application which has some custom features and functionalities, so we want to modify those APIs as per the application needs. Is it possible in any way like Shopify has any Enterprise program solution which we can purchase and modify their APIs?
You can't do that. But you can build your own API as a proxy. Your API would rely on Shopify API and enhance the models, functionality, etc.