2015 Altima no start key fob issue - Help!

When you put the fob near the button, it is read by a near-field radio which is a completely different system.

There are three types of communication:
1 outside the car to unlock the doors
2 inside the car (e.g. driver's pocket) to allow starting
3 near the button as a reserve way to allow starting

I don't know if 1 and 2 use the same radio system or not.
Not sure I understand your question, but the car is 100% stock. No aftermarket radio.
Fobs work through radio waves, various kinds of radio waves. As others said, since you only have one fob it's going to be difficult to isolate the issue to the fob side or the car side.