Search the Elgg docs for webserivces. Core uses RPC-XML approach.
Alternatively this should still work in 2.x https://github.com/hypeJunction/hypeGraph

thank you Ismayil,
I understood that to install hypeGraph is necessary another module.
Do you know if some good responsive theme is available ?

Thank you 

Not to sound like a complete dweeb but I made both Android and Windows apps by just having the apps load the url of my Elgg site. I use the aalborg_theme and the site loads responsive in mobile, or desktop. Works excellent. Not much as far as groovy programming but it works and you could alter the url or whatever you wish to load. I use Android Studio for Android apps and Visual Studio 2017 for Windows..if I can ever afford a current Apple Mac and the Apple Developers fees I will make Apple apps as well in Visual Studio linked to the mac.

https://play.google.com/store/apps/details?id=com.ablight.app

https://www.microsoft.com/en-ca/p/abrahams-light/9p54ndkgh9zp?activetab=pivot%3aoverviewtab

Hi Herb, 
Thanks for the help. 
I've noticed that  aalborg_theme is for elgg 1.9, I've 2.3 installed. Do you think is working ?

So you create and android app using the activity that call the external url right ?
Best regards
Ivano

Aalborg theme is a bundled plugin of Elgg. Of course it still works on Elgg 2.3.

Hi Iionly, 

tss you are right i was not reading well the pglugin names hahah
thanks

Yes, I am using Elgg 2.3.8 and aalborg works fine. I am sure there will be a good responsive plugin for Elgg 3 as well. Yes you just call whatever url you wish.