I haven't figured out yet (I've done it before but don't remember how) how to transfer a file from my Kubuntu PC to my Android phone (that has KDE Connect). Can someone explain this process? I think I'm missing some software in the KDE Connect in my phone. Thanks.

If your phone and PC are paired in KDE Connect., then you will see the phone on Dolphin's sidebar. There should be an option to send to a device by right-clicking on a file as well.

Are the two paired? Are they on the same network?

Thank you for your reply, especially so quickly.
Yes, my phone and KDE are paired, and the phone is in Dolphin's sidebar along with two others.
And, yes, right-clicking a file does show a "Send via KDE Connect" item in the context menu, with a popout sidebar showing all three identical Moto X4s connected but not distinguished from each other. Clicking the first and last yields a KDE notification popup from the system tray saying "Sending to Moto X4," but the progress bar shows no progress; it's just gray & has no moving line segment. Clicking the middle one yields that notification popup, & its progress bar shows a green line segment vacilating back and forth across the progress bar as in a waiting mode. On both the computer's KDE popups and the phone's notification section (at top) there shows nothing about a file transfer. On the third computer KDE popup display, the one from the middle one of the three in the context menu's send via KDE Connect sidebar, the one showing the moving/waiting line segment, nothing happens; the line segment just keeps running back and forth. And that phone's notification (at top) shows two notifications saying "Failed receiving file from lynn@lynn-All-Series."

It's looking more and more like my phone's barred from using my WiFi because a download of Orfox to the X4 I normally use is also prevented. Yet all phones show active WiFi displays. Is it possible that my having installed Orbot on the phone is preventing the transmissions?

I need to simplify the above post and conclude it. I've since found out how to label & thus distinguish the three identical phones and have done it. So, now I can tell the difference everywhere they show up. And, after running a large bunch of Kubuntu's updates and rebooting, then restarting KDE Connect & repairing the main Moto X4, I was able to transfer my files from the PC to the phone. Problem solved. But I still don't know what the orginal problem was, why the files wouldn't transfer.