Not finding the latest version of MacOS to download
I have an iMac (Retina 5K, 27-inch, 2019), and it is running macOS Big Sur 11.7.10. When I plug in my iPhone XR running iOS version 18.1, I get a notice that I need to install a software update. I click on install, but there is no update to install on my computer. I see on the list of what computers are compatible with what macOS that my version of the iMac should be compatible with the latest OS, but even when I go straight to Software Update it is not finding the latest version for me. What can I do to update my OS?
iMac 27″, macOS 11.7