Nvidia still use monolithic die in their GPU whereas AMD use chiplets. I wouldn't think there is a great variation in costs. Nvidia can use four templates for all their families and release cut down versions as they get more failed dies. More chance of fails with large dies.
They all use the same factory to make all the chips -TSMC.
Its more profitable for AMD to use the silicon to make a Ryzen 7700 than it is to make the GPU.
If they didn't have the foothold in Consoles it would be a good question as to why they make GPU
stacked 3d cache = 2 layers of 96mb each of cache on one CPU, right not the 9980x3d only has one layer. If so the PS6 is based on AM6 and we won't get CPU based on it until 2026 at earliest.
AMD can probably use the graphics core chiplets in other formats, such as in their APU and mobile processors, given AMD don't have as many GPU to use them in as well. Only making 9070 and 9060 this gen. 4 cards so far though some people wish they make a 9080 XT as well.