I had this same issue today - after updating, so I thought I would just document what I experienced and what fixed it.
My device: Pixel 7 Pro
On some updates, the "apps updating" or "optimizing apps" takes quite some time. So I see that as being a normal thing. I didn't realize this was still ongoing when I tried to open the camera app to scan a QR code (link). The camera app opened to a black screen, and then the app stopped shortly thereafter.
When I checked notifications, I saw that apps were still being "optimized", so I figured maybe the camera app itself or some software component was in the process of being updated. I waited.
Once the apps were all optimized, I tried opening the camera app again, but still no joy. Same result - black screen.
I checked this thread for assistance and more info. I see this (often) being dismissed as being "hardware related". However, I also lean toward this NOT being the case as it seems to only occur right after an update. With that said, I don't know why it happens.
SOLVED (how):
As others stated, I tried completely shutting down the device. I did not "reboot", I shut down. I wanted the power completely off. I then turned it back on. And after letting everything start, I tried the camera again. Worked perfectly.
Why does this work? No idea. Theory? Maybe trying to open the camera app during app optimizing places the state of the camera app in some condition where it can't start until the OS is shutdown and restarted? Would love to know why myself so I know best practices for during/after OS updates.