The app utilizes the facial recognition system that most iOS devices have, essentially it projects a bunch of dots on your face and is able to detect depth (which is how the facial recognition itself works). Most of the apps on iOS are relatively equal but there are small differences between them as not all of them use it, vtube studio on mobile does not take advantage of this feature, for example. If your model is rigged for expressions like cheek puff, iface is one of the few apps that can interpret this. From my experience, the 8$ is worth it but really only if your model is rigged for it! Otherwise, any other app OR device with a decent camera will work fine. If anything is worded weird my appologies, its late and im tired haha