Smartphones as a general good are a life necessity at this point, but esoteric offerings among them like iPhones or Sony Xperias or the higher end Samsung Galaxys or Google Pixels are luxury goods because nobody needs the features brought by the much bigger price tag.
It's like how a bottom trim Toyota Corolla or Honda Accord is a necessity because you probably need a car, but anything beyond it is a luxury because whether you want that car is different. Another example would be choosing name brand foodstuffs at the grocery store instead of the store brand, that's also choosing luxury goods assuming the store brand is cheaper.
[1]: https://www.samsung.com/us/smartphones/galaxy-a15/buy/galaxy...