How to move files between Ubuntu and iPhone
On recent iOS - you can just share out the files over SMB and use the files app to mount the share / move files and folders.
For doing it without a network or file share, here are the older steps using open source libraries to do this without needing to jailbreak the device.

http://www.libimobiledevice.org/

If you don’t see this included, consider using homebrew to install that package and the http://askubuntu.com people are amazing resources if you run into issues.

https://askubuntu.com/a/704677/21224