Good morning.
A FTP transfer to a present SD Card would be fussible and the PLC has to update then via the application. The main purpose is an automated update process can you imagine to update > 100 remote PLCs with UniLogic? And we are on a mobile projects so it might be possible to not have a stable internet connection. So it would be more secure do download the SW first to our edge device and start then the process.
We do this with our own electronic in multiple ways (wifi, LTE-> send to microcontroller, store locally, ask user/or even no confirmation on some projects and update)