From what I understand some fighters had virtually no armor (say the A6M Zero) or self-sealing fuel tanks, but other fighters had a armor plates (say behind the pilot), more rugged construction (say thicker metal on the wings, fuselage, "armored" windscreens [I take that to mean thick glass?], etc).
Were there typical placements of armor plates protecting the pilot or did it vary considerably? What are some good resources for looking into this?
I know the question is pretty broad, so apologies for that.
For the most part there was very little armor included in fighter aircraft of WWII. The Japanese Ki-43-II only had a single 13mm steel plate behind the pilot. Even planes renowned for their ruggedness such as the P-47 had only minor armor. The P-47D had a 10mm plate behind the pilot and a small plate in front of the pilot under the canopy. For fighters, armor played only a small role in the durability of the plane. Fighters had to be light and manoeuvrable in order to effectively fit their role. The Bf-109 came with an additional armor plate behind the headrest, but this was often removed by pilots who considered increased visibility to be more valuable.
The only other protection resembling armor on fighters would be strengthened glass which could withstand a few hits from low calibre rounds. This would only be found right in front of the pilot, with few exceptions. The skin on the wings and fuselage were not thickened because the disadvantage of the extra mass far outweighed any small increase that may have been gained in this manner.
If you would like to know more about planes with heavier armor we would need to look at ground attackers such as the legendary IL-2 and the Hs.129. A modern example of such a plane would be the A-10 Warthog currently in service with the US military. I would suggest you watch this video by youtuber Bismark for a good overview of this topic.