Catalytic Converter - can someone explain this to me?

I have a few questions about the catalytic converter(s) for a 2009 Aveo5 hatchback.

So, apparently there are two of them, one in the front under the heat shield, another under the car on the exhaust pipe.

The products that I see online seem to indicate that the front cat has 4 branches (cylinders?) coming out of it, it's a pretty decent sized part. I'm really, really new to car repair, past basic maintenance, I'm not sure what everything is called.

But in the video that I watched, it seems like the guy just took off a piece of that, I've attached a picture with what I'm talking about circled, and a link to the video.

So do you have to replace this whole thing? Or just the part he took off? Am I not seeing it right?

Also, I'm a larger girl that doesn't really fit under my car without jacking it up, so I can't just crawl under my car and check. Are the original rear cats (on the exhaust) typically bolted on or welded on? I'd assume bolted, but, you never know.

https://youtu.be/Q7qHxv57-ys. - video I watched

Pic of what it looks like to me he removed, circled.
Attachment 11696

Thanks in advance!