I'm trying to find the folder with the BBC demo in so I can copy some short apps from my laptop and try them on Android. I'm using ES File Explorer, but can't find the the folder
The Android folder containing the supplied example programs isn't ordinarily visible from outside BBC BASIC. Should you want to, you can always obtain those programs by downloading the Windows, Linux or Raspberry Pi edition of BBCSDL and extracting them from the Zip.
As far as your own programs are concerned, you should store them in BBC BASIC's '@usr$' folder which is (or at least should be) visible from ES File Explorer (or directly from a PC via a USB link). You could then transfer them into the folder containing the example programs, using BBC BASIC's 'IDE' commands, but really there's little point.
As I've remarked previously, the '@usr$' folder is likely to be here, or in the vicinity: Android/data/com.rtrussell.bbcbasic/files/.