Unlike the rest of the world, there's really no all-in-one CSC for Unlocked US devices (unless you count XAA, which is used for non-US SIMs or carriers that don't have their own CSC, like Google Fi).
So for example I'm using an AT&T SIM in my S21+ Unlocked. The phone loads the "ATT" CSC just like a normal AT&T branded device would. The only difference is that there's not bloatware and updates come from Samsung directly.
Samsung doesn't usually release Unlocked updates separately because the updates still need to be approved by carriers. Get it?
As @productfred mentioned even for unlocked devices, we need approval from the carriers as they still need to follow carrier requirements when camping on their network. This delays process as we need approval from multiple carriers and it may be best interest of carriers to prioritize a carrier locked device testing before unlocked device testing as majority of their consumers are on carrier locked devices.
One UI Beta Team