So recently I’ve noticed under my 2004 g35x that the pipe has rusted away from the muffler. After tons of searches I’ve honestly confused my self on what it even is. If I got a 2005 coupe midpipe is this the pipe that would replace the one in the picture that’s rusted or is the rusted pipe connected to the mid pipe? In that case is the rusted pipe just a resonator? Sorry if that didn’t make much sense.
edit: is this pipe even stock? After looking at diagrams and pictures the rusted pipe doesn’t look OEM at all

 

I agree it doesn't look OEM, it should have some heat shielding on it.

There are 3 sections that make up the exhaust after the catalytic converter, the cats go to a "y-pipe" that converge via bolted flange into the single "midpipe" that have a bolted flange connecting it to "axle back / muffler".

It looks like you rusted out the midpipe right at the bolted flange to axle back muffler.

 

Whatever midpipe you are using it looks like it has a resonator added to it.

Go to a local reputable muffler shop and see if they can fix it, if the rest of the midpipe is fine they can weld on a short section of new pipe and a new flange.

They will also charge you for a gasket if the existing one cannot be reused, imo this would be a < $100 repair total of the midpipe can be reused and fixed.

 

Likely the resonator was added for a reason, the exhaust may have some ear splitting drone without it so keep that in mind if it cannot be fixed.