Plugin your iPhone via USB cable, then run the following commands:
# install required packages
sudo apt install libimobiledevice6 libimobiledevice-utils ifuse
# pair device
# note: maybe interaction like entering the access code on the phone is required
# rerun this command after entering the PIN until you get a success message
idevicepair pair
# enable multiple connections
usbmuxd -f -v
# create mount point
sudo mkdir /media/iphone
# mount your device as userspace filesystem (FUSE)
ifuse /media/iphone
# open files in favorite file explorer (Desktop OS only)
xdg-open /media/iphone
# unmount phone after file transfer
fusermount -u /media/iphone
# alternate method: ifuse -u /media/iphone
There is also a command-line tool to download photos from iCloud. See https://github.com/icloud-photos-downloader/icloud_photos_downloader