but how can I judge whether it is extendable by CHDK-like program, whether it uses a file(VERS.REQ) to show extra info, and which key do I need to press to load VERS.REQ?
an then how to program?
These are specific to canon. Other devices may or may not have similar things, but they won't have the same name or exactly the same functionality. If such a thing exists, no one is likely to guess it without additional information. It's possible that the specific OS has something standard, but you haven't mentioned which OS.
You need to somehow get a copy of the firmware of your device, and reverse engineer that. The simplest way to do this is to get a firmware update, but Chinese offbrand manufacturers frequently don't issue updates. If no firmware updates have been issued for that device, maybe you can find an update from a similar model from the same manufacturer (or occasionally a different brand that is actually the same device). Failing that, you are probably SOL unless you can either find some backdoor functionality or have access to a well equipped lab.
For phones, I would suggest getting one that has an open firmware and allows you to load unsigned images (
hello google). Or depending on your requirements, an SDK that runs on unrooted phones may be sufficient.
