That is usually how people upload edited file. When i install mods i download all the files needed using ftp, copy the files so i have a backup and edit 1 set of the files then i upload all the edited files again using ftp.
It is just a figure of speech when people say upload the files. It doesn't mean they sit there in file manager and upload them for ever and a day.
Some people like to use their actual editors on their computer, it's faster and if you internet connection drops while saving a edited file on file editor you could lose everything but if you do it via FTP you are safe.