Most DRAM is Micron (US, with some fabs spread elsewhere), SK Hynix (South Korea) and Samsung (South Korea, with some spread IIRC), but there are a few other smaller players for specific technologies.

CPUs are mostly Taiwan, yes, but Intel has fabs spread all over the world. So do players who make smaller, embedded stuff.

NAND flash is spread out, too.

Taiwan is very central, but not for memory, really. And China has mostly just done the PCB/lower tech assembly until recently.