I don't think it's for OS in the way most of think it is. That would mean it would have a split memory pool like XBSX, which Cerny doesn't like. Which means the CPU doesn't benefit from that 2GB DDR5.
On PS4 Pro, it was separate form the main chip and acted as cache.
The nitty-gritty details of the PS4 Pro, according to system architect Mark Cerny / More power, at what cost?
It also makes use of its system memory in a more efficient fashion, using a higher bandwidth on the 8GB of GDDR5 memory to squeeze out an additional 512MB for game developers to make use of. An additional 1GB of conventional DRAM was put in the Pro to help it switch applications, so software like Netflix will be stored on the DRAM when open but not in use.
Here we can see the PS4 Pro's motherboard, which shows the 1GB DDR3 connected to the SCEI I/O controller (Auxiliary processor) and not the main chip.
PlayStation 4 Pro Teardown
Same for PS4 as well, but with 256 MB of DDR3.
PlayStation 4 technical specifications
Auxiliary processor
The PS4 includes a secondary ARM processor (with separate 256 MiB of RAM) to assist with background functions and OS features.
On the PS5, we can see a similar approach. 512MB DDR4 is connected to the SSD controller as cache.
PlayStation 5 Teardown
If Sony is following what they did with the PS4 Pro, the 512MB DDR4 would increase and change to 2GB DDR5.
PSSR and Ray Tracing memory utilization is probably the reason they went with 2GB.
The PS5's SSD controller actually handles a lot of stuff.