Adjustment of Status - I-485

Question regarding the background checking and finger prints , if someone can shed some light on it please . In our 1-485 case the receipt date 11/29/2016 ( through the Miami FL office ) shows that " Fingerprints fee was accepted " on the same date 11/29/16 it also says that the " Our National Benefit center location is working on your case " . Does that mean they start a background check at that time , is it the NBC the one which works on using the fingerprints background check ? Or is it only that the fee is accepted ? it took us almost two years after the receipt date to have our interview , a month after the interview we received a letter of approval for the 130 , but still waiting for the 485 ( one line still stuck on interview completed and case must be reviewed eve for the 130 which was already approved ) question is , do they now use the fingerprints for a background check ? two years later or has anything been done about the fingerprints/background check while waiting for the interview ?