You may or may not see a Relocated Items folder on your desktop after a macOS upgrade.
When you upgrade your Mac to macOS Catalina or Big Sur, all your files and data are carefully reviewed to ensure they’re valid and authorized, undamaged, and in the correct locations. If some of your files couldn’t be moved to their new locations, they are placed in a Relocated Items folder. You can find the Relocated Items folder in the Shared folder within the User folder. The PDF document in the Relocated Items folder explains what to do about files placed in this folder.