cross plat form application - how does it work

Question

I am developing a billing software with a centralized database unit .

The question which i have is , in restaurants they used to take orders either in android device or mac device how does the order gets sync with the one in the billing counter . If its basically via the android service or mac service , can you guide me where can i get tutorials.

 

 

P.S : i am building desktop application with windows forms and mobile app via xamarin only for android as for now

 

Thanks in advance