My Trash automatically deletes files, big sur.

It would help to know what command you used.

Regardless, this might resolve your problem.

Open you home folder in Finder (cmd-shift-h)

Type this shortcut: cmd-shift-. ← that is a period at the end of the command. Or, I guess you might consider it cmd->

That will show the hidden files in your home.

Find the Trash folder and Move it to the Trash. You may get requested to Authenticate to remove it.

Empty the Trash.

Try dragging something into the Trash (that will cause the OS to create a new Trash folder in your Home).

You can type cmd-shift-. again to hide the hidden files.

Note that on some keyboards, that command does not work. If it doesn't show the hidden files, there is another way. Copy/paste the following into Terminal:

defaults write com.apple.finder ShowAllFiles TRUE

After you are done, use this to hide them again:

defaults write com.apple.finder ShowAllFiles FALSE

What do you mean by “automatically?”

There is a setting in Finder preferences to delete items over 30 days old.

If you mean that it deletes them as soon as you move them to the trash, that usually implies there is a problem with the Trash folder permissions. Have you noticed any other permission related issues in your home folder?