> Where do you get the FTP client and server from without an Internet connection
Until last year, your web browser will usually download files. These days, you have to start an HTTP server. Which is actually easier, because FTP is a messy protocol.
> how do you create a network between the two
WiFi Direct has been built into phones for at least a decade.
> And would you say that this is comparable in terms of complexity with e.g. Airdrop or selecting "send via Bluetooth" and picking the recipient's device name?
On the sender side, the "send via" option can appear in the standard sharing app list. On the recipient's side, you may need to scan a QR code to finish the WiFi Direct connection. Starting the download should be as easy as having the sending app pretend to be a WiFi login portal so the phone automatically pops up the web page.
But if you're just sending files offline, there's always Nearby Share or plain old Bluetooth if the files are small enough.
> On the sender side, the "send via" option can appear in the standard sharing app list.
Can, but does it? Defaults matter. And how would the receiver be notified that they should download some file via FTP?
As long as such a feature does not come preinstalled on smartphones, I'll continue considering this lack of previously commonplace functionality an intentional vendor lock-in.
>how would the receiver be notified that they should download some file via FTP
You tell them "hey I want to share a file with you, scan this QR code". The QR code sets up the WiFi Direct connection, the fake portal login should auto-open a "click here to download" button (or redirect to a file, but I don't think that's allowed), and once the file is downloaded you terminate the Wi-Fi Direct connection.
But that presumes phones don't have a method to share files. They have that feature built in. It's called Nearby Share on Android and AirDrop on iOS. Both use some weird pairing mechanism and some form of Wi-Fi Direct to transfer files, but the interface is just a bit more integrated.
When Apple opens up Airdrop or when Google convinces Apple to pre-install Nearby Share, we'll get the universal file transfer we desperately need.
Until last year, your web browser will usually download files. These days, you have to start an HTTP server. Which is actually easier, because FTP is a messy protocol.
> how do you create a network between the two
WiFi Direct has been built into phones for at least a decade.
> And would you say that this is comparable in terms of complexity with e.g. Airdrop or selecting "send via Bluetooth" and picking the recipient's device name?
On the sender side, the "send via" option can appear in the standard sharing app list. On the recipient's side, you may need to scan a QR code to finish the WiFi Direct connection. Starting the download should be as easy as having the sending app pretend to be a WiFi login portal so the phone automatically pops up the web page.
But if you're just sending files offline, there's always Nearby Share or plain old Bluetooth if the files are small enough.