I am setting my flight scedule for a trip to Buenos Aires and was wondering where I will have to go through customs if I have one domestic stop before leaving the US. I am using the same airline company for all flights. Will I go through costums at the first airport or at my layover?
On your way to Buenos Aires, you will only go through Immigration and Customs upon arrival in Buenos Aires. There are no border formalities leaving the USA, other than having your passport checked by the airline at check in time.
On the way back from Buenos Aires, you will go through US Immigration and Customs in the first city you land at in the USA, which would be Atlanta based on your posted routing.
There is no exit customs or immigration check when you leave the USA. You will therefore do this neither in Portland nor Atlanta.
The airline will, however, pass your information on to the US immigration authorities. Whether they gather this information at your initial check-in or at the gate on your transfer probably varies from one airline to the next. I suspect they will check at your initial check-in. This won't have much of an impact on you, however.