I use WeChat to talk to my in-laws in Mainland China. I am aware of the mini-apps that exist but they are severely lacking features & more of add on utilities to the WeChat interface.

For e.g. a popular one is a mini-app for DiDi cabs. It'll allow you to book a nearby cab & place a call/text using the WeChat integration. It is more of utility to WeChat/Weixin rather than an appstore within their app. Agreed it is somewhat borderline in definition, but the way I see it - the utilities and apps are sandboxed within WeChat ecosystem.