A) The people you friended uploaded their contact information
B) People searched your name on Facebook
C) Facebook creates ghost profiles with all these details until it has enough confidence to sync it with an new/existing Facebook profile
B) People searched your name on Facebook
C) Facebook creates ghost profiles with all these details until it has enough confidence to sync it with an new/existing Facebook profile
Once an EU citizen contacts them and requests the dump, they get redirected to the same tool. You have to explicitly bug them and quote the article 15 (of the GDPR) a few times to receive a different dump.
Disclosure: I haven't done this personally, but I have talked to people who did and read their interactions with Facebook. I can't guarantee that this info is still up-to-date, since that happened months after GDPR went into effect.
That data is in blackbox ML system which is exclusively for ad targeting, and it can't be described :) that's the Facebook's defense.
GDPR doesn't cover the intermediate info AI/ML generates on you.