The December updates for Pixel 7 and Pixel 7 Pro are available to manually download on Google's website [0], so the updates do exist, although Google might not be rolling them out to the general public quite yet. But the December update for Pixel 7a are completely missing from that website, and trying to update from the Settings app also shows no updates available.
There are instructions at the top of the link. You need to use "adb" from the command-line on a computer, but it won't wipe any of your data, so it shouldn't cause any data loss. If you don't want to use "adb", you might be able to use [0], but I haven't tested it myself.
[0]: https://developers.google.com/android/ota