Secure ShellFish is a SFTP client that lives in the Files app on iPad and iPhone. Access the directories on your Mac, Linux server or Raspberry Pi like you access Dropbox and iCloud Drive. Files are downloaded when opened with changes uploaded as you make them or as soon as possible if you temporarily lose the connection to the server.
It supports the Shortcuts app allowing you to automate file transfers and can be used from the share sheet for fast uploads.
The app is free to download with pro features requiring a single permanent in-app purchase.