After Monterey upgrade, all my files are gone!

What version of the OS did you upgrade from?

Where were those files before?

This seems to indicate that you used to have files at the top level of the drive, and not under your home folder.

That location is not allowed anymore (and with good reason), so an upgrade would do what you have seen.

All your files are under you home folder, which is inside Users.

Just Command-N to open a new folder, and it will (probably) show you your home folder.

Yes, it is a security measure.

Since Catalina, there are two separate volumes: one for the system files (by default called "Macintosh HD"), and one for user documents and applications ("Macintosh HD - Data"). This separation is more or less invisible for users: the Finder makes it look like there is still only one volume.

Since Big Sur, this has been tightened further: the system volume is sealed and always mounted read only.

This makes it so system files cannot be tampered with, and that is why not only you can't write there, but what was there before the upgrade was moved elsewhere.

To separate system from data. The default data folders are separate so you can upgrade, or reinstall the OS, without touching the user's data. This is by design; to put them anywhere else other than the default folders would have taken intentional user intervention.