APP

Hi,

What kind of app are you trying to make ? For customers to purchase on your website ? For you to manage your products ? Your orders ? Something else ? Could you tell us more about your project so that we get a clearer picture of what is needed ?

Hi,

Then, I suppose that there is one main thing you need: loading and updating the products data from the website to the app.
One thing you could do would be to configure the Google products plugin to generate a XML of the data of the products and then have the app retrieve that XML and process it, once in a while.
For the payment, the best would be to go through the payment capabilities of Google Play and Apple's app store. That way, it will be a lot more seamless for the user than going through your website inside the app.

There are several ways to make it possible for an external app to be able to authenticate users of a joomla website:
stackoverflow.com/questions/2176595/joom...on-from-external-app
docs.joomla.org/J3.x:Creating_an_Authent...on_Plugin_for_Joomla

Then to register a new user from the app, you would have to create a plugin with code like this:
forum.joomla.org/viewtopic.php?t=988179

As you can see, these are Joomla problematics, not really HikaShop problematics and joomla doesn't have ready made solutions you can leverage as far as I know.