Smartbracelet

Robby's smartbracelet (top) versus Hatchi's smartbracelet (bottom).

A smartbracelet is a slim, innovative device usually worn on one's wrist, although it seems to come in different variants as Yang appears to wear his device as an earring. It has a variety of functions, such as contactless electronic payments, money transfers, browsing the web, projecting a holographic screen with images (or even a full holographic 3D model), taking photos, enabling audio calls, and an in-universe equivalent of e-mail and messaging.
Smartbracelets can be deduced to be waterproof, as characters are multiple times seen showering and bathing with them on. Because of this, it can also be assumed that they are, in general, worn constantly without taking them off on any occasion. Similarly, they are not damaged by electric discharges. Another assumption is that older models are thicker in width: Robby has the thickest smartbracelet out of the whole cast, and when it malfunctions when he tries to pay he says it's because it's damaged, perhaps due to its age. While it's likely he just doesn't have the funds, the seller who seems to know him well doesn't question the damage, and accepts this explanation. In turn, Hatchi, Allo, and Gras all have very slim bracelets, presumably because they can afford a newer model.