So, Duty free area exists after your immigration counter in most airport.
This is the area where you've checked out from country A, but yet to check in in country B. So, technically the VAT / GST charges of either country A / B do not apply here, from legal point.
People mostly buy alcohol and expensive chocolates for cheap rates here.
In my case, we bought the phone while travelling to Vietnam from Mumbai Airport. While returning also, we asked if we could buy an iPhone , the store owner said they can't sell mobile on Arrival duty free area, only at departures.