Comment by ChrisGreenHeur

It just depends on what architecture your computer has.

On a PC, the CPU typically has exclusive access to system RAM, while the GPU has its own dedicated VRAM. The graphics driver runs code on both the CPU and the GPU since the GPU has its own embedded processor so data is constantly being copied back and forth between the two memory pools.

Mobile platforms like the iPhone or macOS laptops are different: they use unified memory, meaning the CPU and GPU share the same physical RAM. That makes it possible to allocate a Metal surface that both can access, so the CPU can modify it and the GPU can display it directly.

However, you won’t get good frame rates on a MacBook if you try to draw a full-screen, pixel-perfect surface entirely on the CPU it just can’t push pixels that fast. But you can write a software renderer where the CPU updates pixels and the GPU displays them, without copying the surface around.