If I download a VPN app in South Africa onto my smartphone and laptop, can I use that VPN in China as well, or only while I am in South Africa?
Very good question Sanet. You should be able to use a VPN worldwide. They're not limited to any one specific geo location.
While you're in SA, you may find that connecting to a certain VPN server works faster than others based on how close they are to SA. As you travel, you'll have to experiment to see which server works the fastest when connecting.