For the sensor it seems like some trucks have 2 pre o2 sensors and some have one. It's something I dealt with when I needed a o2 sensor. I Think they switched mid year for 01. I got one with the single o2 sensor before and after the cat. I got a NTK o2 sesnor from Summit and used a o2 simm from The Fastman on the rear. Seems to work well.

 

Trucks with California Emissions should have 2 upstream O2 sensors.
Some will also have 2 downstream O2 sensors.
It depends on year.
Most will have 1 and 1 and yes the parts listing is confusing...

 

The O2 sensors are only covered under the original 3/36,000 warranty...