I just entered the USA without passport control at Atlanta airport

You went through US CBP Preclearance in Toronto before boarding your flight. This is US immigration and customs, with US immigration officers stationed on Canadian soil, and unless something unusual occurs, that means your flight arrives in the US in the same way as a US-to-US domestic flight, with no need for any further procedures to enter the country.

Since you're a NEXUS holder, your trip through immigration was short and simplified, as the US considers you a trusted traveler with no need to see an officer unless you have something to declare or are selected for further scrutiny.

As for your second question:

but how the hell did the security know if i already precleared at toronto airport and openned up the door to separate hallway ?

Everyone on your flight went through preclearance, because that was part of the process before you could get to your boarding gate in Toronto. The airline's staff in Atlanta knew you were on a preclearance flight and opened the appropriate doors to direct you into the airport concourse instead of the secure corridors to the immigration hall. If you had arrived in the US on a non-preclearance flight (say, a flight from London), they instead would have ensured that your only path was to the immigration hall, and you would not be able to leave the airport until you went through immigration and customs. If the airline staff does somehow screw that up, there could be substantial fines for the airline and a great deal of trouble and inconvenience for everyone (as occurred during this incident in Chicago).