IIRC they built the models with user data, but stopped collecting from phones because so many different sensors are used for rH and temp in phones plus being in pockets, battery temp, etc meant the data had too much variance. Even though I hate that they did this bc it's my favourite weather app, it actually would make sense to just use iphones if they were going to start collecting that data again since all the hardware is the same.
The barometer data that comes from phones (both Android and iOS) is extremely resilient and there is a lot of value to be derived from it. Ongoing research by Cliff Mass and team (at UW) shows that machine learning can help bias-correct and error-correct barometric pressure data on the fly with very good results. That data can also be assimilated into WRF to make forecasts with higher accuracy.
