While much of the iOS is Open Source or Free Software, the end-user can't really inspect the source that went into their particular device.
Note that, to the extent that the CIA steals Apple secrets from foreign sources, they may not be violating any US laws. However Apple does the vast majority of its system software development in Cupertino, California.
My next phone is going to be an Android that enables me to completely replace its firmware binary. While this does void the warranty, rather than having to be concerned about potentially skanky jailbreak exploits, one more or less just sets a flag with an Android SDK command-line tool, then you can install your own firmware.
That will be of limited use in protecting me against hardware backdoors, but at least it will let me pretend that America is still The Land Of The Free.
Among the reason I am so adamant about stuff like this, and that I use my real name when I post about it so publicly, is that I am related to Roger Sherman, the fourth signer of the US Declaration of Independence, as well as to William Tecumseh Sherman and George B. McClellan, the two Generals-in-Chief of the Union Army during the American Civil War.
My mother was an active member of the Daughters of the American Revolution; I myself am entitled to membership in the Sons of the American Revolution.