You see, that’s why xkeyscore is illegal.

Ummm, "illegal" does not equal "not used". It just means "not disclosed, and obfuscated through parallel construction".

I mean, do you really think that a military intelligence agency operates under domestic law? And from what I've read, need to know structure within the NSA doesn't map to command/reporting structure. So there may be components that operate ~independently as deep backup.

As far as the legal situation is concerned XKeyScore was a foreign intelligence tool, domestic surveillance does still require a Fisa warrant and I don't think there was ever evidence that XKeyScore or similar NSA tools were used as tools of mass surveillance against American citizens.

It's been a while since I've read up on this, but my understanding is that the NSA vacuums up everything it can and operates under the rather creative legal theory that this doesn't count as "collection" until it's returned by the search engine, which they pinky-promise won't return it unless their prediction model is at least 51% sure that it's a foreign communication.