Exhaust

I’m looking to put a exhaust system on my 2015 Jetta tdi and I wanna know if you have to do a def delete in order to do the exhaust

It's called a cat back system for a reason.

Look in to Bank's Power 3" cat-bac. you'll notice a difference and better mpg (thats the only muffler/restriction) since backpressure is highly minimized. you can do this and keep emissions or until you save up.
To keep emissions: kerma cr190 kit, stock airbox but different filter i.e. KN washable or afe filter cone options, banks 3" cat-back.
To go unrestricted: rawtek, kerma, enjoy (or cut the banks reducer tip and weld the 3" section for the only 3" straight through) - is what I did.

Piggy Backing

Curious as well, however I'm asking about the 2015 passat b7 with the cvca (same block as CUNA, CUPA - golf variants of same ea288 motor gen 3, same oval port - looked rectangular really)

I'm finding that exhaust fabricators are either sticking with 2.5" thru or 3" mid to 2.5" cat-bac/exit. is there a reason for this i.e. velocity increase to minimize backpressure because of longer piping or am I better off figuring out how to do a straight 3"?

magnaflow has one (3" resonated and muffled cat-bac) for the petrol engine but I don't trust the backpressure albeit turbo petrol or not.

running straight from turbo via rawtek and malone, asking because I am going to be back running a gtd1449vz turbo finally. valid option for jetta available, jack for passat as far as 3"cat-bac (i like straight thru)

A 2.5" full exhaust is plenty big enough for all but the most extreme tdi builds. A cat back alone will do nothing for power, fuel efficiency, or sound. It can lighten the car ever so slightly, and lighten your wallet more significantly.