If every NFL team tied, It would depend on which games they won, how much they won by, etc. The NFL has an elaborate tie-break procedure for these sort of incidents.
Every team plays 6 divisional games, 6 conference (not-division), and 4 non-conference games.
Within a division, the first tie-break is divisional record. So, if one team went 6-0 in the division, but 2-8 in all the other games, they would be the division winner.
There are actually 12 levels of tie-breakers, though historically it's never had to get past the fifth level (Strength of Victory) for any playoff berth.
Ultimately, if all 256 games finished in a 3-3 tie, the playoffs would be determined by coin flip.