Threads, Blocks, and Grids
A kernel launch like saxpy>>(...) doesn't just start "some threads" — it starts a precisely structured hierarchy, and the shape of that hierarchy is what lets the same compiled kernel run correctly on a small laptop GPU and a data-center accelerator with an order of magnitude more SMs. Understanding the levels of that hierarchy, and which ones can and can't communicate, is the difference between a kernel that scales and one that only happens to work on the GPU it was tested on.