On Macs, the architecture is a little different. CPU and GPU both share the same high speed memory, which can run at 800GB/sec. That’s well beyond what DDR5 (64GB/sec) or DDR6 (134GB/sec) can offer. So if you buy a Mac with 64GB, it can use that memory for either system CPU processing or GPU memory.
Out of the box, a Mac will max out at 75% of RAM for the GPU, though this can be adjusted. My 64GB M1 Max runs models that require 40GB+ of GPU VRAM just fine.