I apologize if advance if I missed other posts related to this. I am planning a trip to Istanbul in December. I will fly from Portland Oregon to Amsterdam on Delta then transfer to KLM from Amsterdam to Istanbul. Will I have to go through Customs/Immigration in Amsterdam or will all the customs / Immigration stuff be in Istanbul ?
No customs as you are not entering the Netherlands. And there should be no immigration on a non-Schengen to non-Schengen flight.
As stated above, for this type of connection at AMS, no customs or immigration/passport control. You'll simply get off your arriving flight at AMS in the international, Non-Schengen section of the airport, check the first departure display board you see (scattered all over the terminal), and then walk to your next flight's departure gate (or stopover in a Non-Schengen lounge or dining spot)!
...and while it may be obvious (or not, maybe), you will be restricted to the International Terminal. The only way out is through final destination immigration, in-transit immigration to the domestic terminal, or an international (non-Schengen) flight out.
All of this, and advice from others, is assuming you are on a single ticket from PDX to IST. If you booked the KLM ticket separately, then there may be more to the advice.