But that’s system memory. Not GPU memory. M1 shares that memory, so it’s addressable by both directly, but with ryzen (and almost every other consumer platform) the cpu and gpu memory are separate.
The BIOS doesn't set the shared memory. The BIOS sets the dedicated memory. The shared memory is set by the OS and driver as you need, and the only limit is how much memory you have and how much is used by other processes.
You can force any program to use shared memory by making dedicated memory low. As I said, these programs don't really choose to use it, it's a driver/OS responsibility.
The 5700Gs memory controller indeed can't go above 100GB/s. However 200-400GB/s is not what the M1 Max GPU can do, it's combined performance. You'd have to substract CPU performance. The M1 Max GPU would still be faster of course. But the premise is that GPU performance doesn't really matter.