M1 GPU Cores - Why so few vs PC GPUs ?

What Nvidea calls a 'Cuda Core' is what AMD calls a "Unified Shader" and what Intel calls a "Shading Unit". Each is too primitive to be called a full CPU.

A better comparison is the Nvidea "Streaming MultiProcessor" and the AMD "Compute Unit". The fastest RTX 3090 only has 82 of those, while the AMD RX 6900XT only has 80 respectively. Thats just 25-28% more than an Apple M1-Ultra 64.

With architectural differences, clock rate differences, memory bandwidth differences, etc performance can be extremely close depending on workload.

You asked "or is the threading / kernels architecture completely different in the M1 Ultra chip ?"

Yes, the Ultra chip gets a lot more done w/less cores. It is how the cores operate, not just the number of cores. Apple really does not say much about how the M1 CPU and GPU cores operatel

See:

https://9to5mac.com/2022/03/31/m1-ultra-gpu-comparison-with-nvidia/

Also see comparison with just standard M1

https://www.notebookcheck.net/M1-8-Core-GPU-vs-GeForce-GTX-1080-Ti-vs-M1-7-Core-GPU_10552_7700_10560.247598.0.html