Files can be anywhere, so there’s a big table in a predetermined spot that lists all the files and the place(s) where the file is stored. You can also look there to see what space is in use by files.

When you write a file, the computer looks for a spot that isn’t in use, reserves it, puts stuff there, and updates the table so it shows where the file is.

When you delete a file, the computer simply removes the entry in the table that says where it is and what area was reserved for it. There’s no need to delete the actual data because once the name disappears from the table, it might as well not exist as far as programs are concerned.

Since the space isn’t reserved anymore, the space is free to be used to store other files in the future and will eventually get written over.