In Android, how do I monitor changes made by others (app, internet, bluetooth, dll) to a file or directory
No, in standard android, you don't monitor for activities of others, app, internet etc.
Your program can only track and access your data directory. Others uses its own directories.
But, if you have rooted a telephone, you can write/start a root process or suid process. But this solution is only valid for rooted telephones..
I think FileObserver is what you are looking for