O2 sensor location? (2005)

I've gotten an intermittent P0420 (driver side downstream O2 sensor says there's a problem). Based on other threads, I decided to try swapping out the post-cat O2 sensors to see if the error code would switch to P0430 (passenger side) to identify if I just needed to replace a sensor.

I pulled up an applicable Youtube video:
https://www.youtube.com/watch?v=vEL_xCBELmA

Looks easy. The bulge along the underside of both exhaust pipes is the cat from from what I can tell, and the sensor should be behind it held on with two bolts.

I don't have any sensors downstream of my catalytic converter. The one sensor upstream of it is a single nut with no bolt. I followed it all the way down to where the two exhaust pipes joined and didn't find anything.

What's going on? Am I blind?

 

Can you post more pictures? The exhaust pipe after your cat looks odd. Can't tell if that's just dried mud or something else. It's possible a previous owner replaced that section.

 

Or are we looking at the resonator and not the cat? Perspective is weird. I can’t make heads from tails.

 

While Sue mentions in video she is working on a 4.7(V8), your V6(if profile is correct) is somewhat different as far as CAT and other component locations compared to a V8. Definitely appears some work has been done on your exhaust

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Is the angled piece labeled TWC my catalytic converter? I thought it was the horizontal one (the one that has lines on it in the diagram.
If the cat is the first piece, then the one I have labeled as Upstream is actually the Downstream sensor.

 

The converters indeed are labelled TWC which is an abbreviation for Three Way Catalytic Converter. The upstream O2 sensors are labelled as the two A/F (air/fuel ratio) sensors. Bank 1 is the Left Hand (LH) sensor located on the driver's side. Bank 2 is located on the Right Hand (RH) passenger side. The upstream sensors are located at or near the exhaust manifold flange on either side of the back of the engine. These two flanges are reddish-looking and somewhat triangular-shaped coming off the rear of both sides of the engine.

 

I think this is solved... thank you.

Now just for curiousity what's the second set of TWC bulges that don't have any sensors after them?