In an operating system, when you 'delete' a file or programme, then usually all that happens is that the address of the file is deleted, not the file itself. Thus it cannot now be found.
But the actual file itself is still there, and can be recovered by an 'undelete' command, provided the disk space concerned has not been written over.
This is only an elementary explanation.
There is a type of ballpoint pen that is manufactured whose ink is specially made to be erasable. The pen includes an eraser on the cap. Lemon juice and bleach can also be used.