HHFor users of Android based smartphones, going in and out of China is like going between alternative universes. All the popular apps and Internet sites which outside of China (Google, Facebook, Box, etc) are blocked in China. China Internet companies thrive in this vacuum and are the only ones in the world which have the scale to compete with US based tech giants.
For the average user caught in this battle, it's extremely frustrating. You are likely to have to use different sets of e-mail and cloud based services depending if you are in or outside of China. Forget about Google mail in China. Try using Microsoft mail instead if you want to keep on using a US e-mail provider. Android based Google searches are useless and most people switch to Baidu for search in China.
App stores become a bigger challenge because apps are the basic foundational for getting anything done on your smartphone. For example, when you bring your non-China purchased Samsung Galaxy S6 Edge in China and find that Google Play is blocked, you need to find some alternative to install China friendly apps. Similarly, when you bring your China purchased Xiaomi or Huawei phone outside of China, the China apps you have are less useful and you need to install apps you can't find in the Xiaomi or Huawei app store.
Since it's very difficult to install Google Play for the average user, here are some sites to easily download and install app store APK files. Make sure you first download them using a PC or Mac before transferring them to your Android smartphone.
International Friendly App Stores: Amazon App Store (sorry, Google Play dominates everything else)