Preference Panes no longer open after Big Sur update. Library/PreferencePanes is now completely empty
Make sure you look at the right preference panes folder. There are three of them:

/System/Library/PreferencePanes This folder includes all the panes which are shipped with macOS. It definitely should not be empty.
/Library/PreferencePanes Here you can install third-party panes which should be available for all users of your Mac.
/Users/yourusername/Library/PreferencePanes Here you can install third-party panes which should be available just for &quot;your&quot; user.

Use Finder and select &quot;Go to&quot; – &quot;Go to folder...&quot; and enter one of the path names. Finder will then show you wether it is empty. The last two can be empty, but the first one not.
If the first one isn't empty, try to add another admin user using the steps in this answer and see if this user can open the preference panes.
If it is empty, you should try to install the system again, checking the disk first by booting into recovery mode. Press Command-R during startup to do this.