All those companies sell the "anonymized data" to everyone. for example "ad X shown to user Y targeted on Z at time T" And if you happen know user Y matches u

All those companies sell the "anonymized data" to everyone. for example "ad X shown to user Y targeted on Z at time T" And if you happen know user Y matches user earenndil in a data set you control, now you know user earenndil belongs to demographic Z.