What happens to my OneDrive files if I leave UC San Diego?

The management of your files is up to your department. Keep in mind that the files you upload are associated with your AD account. So if you don't share them with other users before you leave, the files will be inaccessible.

What happens if I delete a OneDrive file? Can you recover it for me?

If you delete a file stored in OneDrive, it will be in your OneDrive recycle bin for 30 days. After that, it is gone. Your departmental IT support nor ITS can recover it for you. There is no back-up.