Oh linked it a little while back, she does a great job explaining UX in general I find. And it's always interesting how the social and material conditions drive these things. For example, this video on how web design in India tends to be very spartan is interesting too. She explains how a lot of the population doesn't have fast mobile phones, and so sites have to be snappy on older hardware which means have much leaner and more functional designs.

It has everything to do with software architecture. You're not seeing the bigger picture here. The architecture is that you have a common UI layer with apps acting as services that plug into it. This doesn't have to be done via an app like WeChat, it could be provided as part of the OS itself. The advantage is that you can mix and match functionality from different apps trivially to create custom UX workflows, and this approach facilitates things like automation where you can make scripts to chain apps together the same way you can do with shell commands.