padding files

Back in the day when I used to use Kazaa there were things called "fake files" and I believe they are still around today as torrents. Basically they file would be bogus but padded to be the same size as the target file.

How exactly do they do this form a technical standpoint? Could you just copy paste a load of nonsense into a text editor?

HEY FUCK FACE, THERE'S A THING IN DAMN TORRENT FILES CALLED A SHA1 HASH, AND YOU CAN'T FUCKING FORGE THAT SHIT, SO EAT SHIT