This is the feature I'm missing the most from KDE Connect, transferring from Computer to mobile, I ended up using syncthing with a 'send_to_mobile' folder, not the main purpose but... it works.
I often transfer files from the computer to the phone using KDE Connect. Right-click on a file in Dolphin > send to device. Many KDE apps also have a similar share button. I don't know how it works in other desktop environments.
Was a bit confused by the fact that you need to input the absolute path to the file, but when I send it I can manage to find where it goes in my device.