This may or may not be what the OP meant but if you trust Google Play and don't trust anything else like an APK from WeChat's website, then you won't be able to get it from the Play Store once you enter China. (Though you won't need to do this if you roam on some carriers like Google Fi or use the WiFi of some nicer hotels)

Yes, that was my point — that if you visit China with a non-Chinese Android phone, the phone will get apps from Google Play. Without a VPN, it won't be able to access Google Play, putting you in a bit of a chicken-and-egg situation. So you need to install the VPN before you visit.

my question was about usage, since for installation you already recommend doing so out of China in the first place

You need to install a VPN before you go to China.

Then, once you're within China, you can use the VPN to access restricted sites and services -- including the various app stores.