If I understand correctly, torrent is a peer-to-peer file sharing where seeders upload file to be downloaded by leechers, and once leechers complete download they become seeders. but what happens if a seeders modify their file? for example what if I download the file, and decide to create a note in that file saying "oi, hey look at me~~~" does that mean that every leechers who get file from me (the seeder) will now download the additional file I just created for fun?
Nope. Once you change the file then you stop seeding since it's no longer the same file.