iOS, as far as I remember, only supports WebDav. Which is useless with an SFTP server, ofc. And yes, there is the solution of installing another file server, configuring it, maintaining it, ripping a new hole into the Firewall, and fiddling around with file permissions, but that sucks, obviously. You could of course buy (or even subscribe to, I believe) a third party, closed sourced, app.
Then there’s Android, with FOSS apps like RemoteFiles, because sideloading.
Or just mount it with rclone.
And in my case, I don’t need an actual mount anyway, because the FOSS Keepass2Android has native SFTP support, because it makes sense to have it.