You can access the Apple App Store in China, but not Google Play. Like all other Google services, the Google Play Store is blocked when in China. This means you won’t be able to access your VPN provider’s app when you’re in the country, so make sure you download your VPN before you arrive in China.
Apple’s App Store is available in China, but it’s heavily restricted and monitored by the Chinese government. Many popular apps like Youtube, WhatsApp, and Facebook aren’t available there as only government-approved apps are allowed. You won’t find your VPN app there either — in 2017, services like ExpressVPN and VyprVPN were removed following an order to remove VPNs from app stores.