How I move files between iOS devices and Unix machines (using SSH)
Suppose, not hypothetically, that you're a Unix person with some number of iOS devices, such as a phone and a tablet, and you wind up with files in one environment that you would like to move to or access from the other. On the iOS devices you may have photos and videos you want to move to Unix to deal with them with familiar tools, and on Unix you may have files that you edit or read or refer to and you'd like to do that on your portable devices too. There are a variety of ways of doing this, such as email and Nextcloud, but the way I've come around to is using SSH (specifically SFTP) through the Secure Shellfish iOS app.
Secure Shellfish's fundamental pitch is nicely covered by its tagline of 'SSH file transfers on iOS' and its slightly longer description of 'SSH and SFTP support in the iOS Files app', although the Files app is not the only way you can use it. Its narrow focus makes it pleasantly minimalistic and quite straightforward, and it works just as it says it does; it uses SFTP to let you transfer files between a Unix account (or anything that supports SFTP) and your iOS devices, and also to look at and modify in place Unix files from iOS, through Files-aware programs like Textastic. As far as (SSH) authentication goes, it supports both passwords and SSH keys (these days it will generate RSA keys and supports importing RSA, ECDSA, and ed25519 keys).
If the idea of theoretically allowing Secure Shellfish full access to your Unix account makes you a bit nervous, there are several things you can do. On machines that you fully control, you can set up a dedicated login that's used only for transferring things between your Unix machine and your iOS devices, so that they don't even have access to your regular account and its full set of files. Then, if you use SSH keys, you can set your .ssh/authorized_keys to force the Secure Shellfish key to always run the SFTP server instead of allowing access to an ordinary shell. For example:
command="/usr/libexec/openssh/sftp-server",restrict ssh-rsa [...]
(sftp-server has various command line flags that may be useful here
for the cautious. As I found out the hard way, different systems have
different paths to sftp-server, and you don't get good diagnostics from
Secure Shellfish if you get it wrong. On at least some versions of
OpenSSH, you can use the special command name '
force use of the built-in SFTP server, but then I don't think you can
give it any command line flags.)
To avoid accidents, you can also configure an initial starting directory in Secure Shellfish itself and thereby restrict your normal view of the Unix account. This can also be convenient if you don't want to have to navigate through a hierarchy of directories to get to what you actually want; if you know you're only going to use a particular server you configure to work in some directory, you can just set that up in advace.
As I've found, there are two ways to transfer iOS things like photos to your Unix account with Secure Shellfish. In an iOS app such as Photos, you can either directly send what you want to transfer to Secure Shellfish in the strip of available apps (and then pick from there), or you can use 'Save to Files' and then pick Secure Shellfish and go from there. The advantage and drawback of directly picking Secure Shellfish from the app strip is that your file is transferred immediately and that you can't do anything more until the transfer finishes. If you 'save to files', your file is transferred somewhat asynchronously. As a result, if you want to immediately do something with your data on the Unix side and it's a large file, you probably want to use the app route; at least you can watch the upload progress and know immediately when it's done.
(Secure Shellfish has a free base version and a paid 'Pro' upgrade, but I honestly don't remember what's included in what. If it was free when I initially got it, I upgraded to the Pro version within a very short time because I wanted to support the author.)
PS: Secure Shellfish supports using jump (SSH) servers, but I haven't tested this and I suspect that it doesn't go well with restricting your Secure Shellfish SSH key to only doing SFTP.