File Transfer From Computer to Phone

I have a problem. It is probably a transition command I don't know so I hope someone can pitch in the solution.

I am trying to transfer .jpg and video files from my computer to my iPhone. I would like to do it by tether, but when I connect, the phone immediately grants access to the computer and shows all the phone files that can be transferred to the computer.

I'd like to know where I can access the command to switch to the preferred file transfer.

I have tried using iTunes, and I get to the point where I have selected files to transfer from the computer, and at that point I can't figure out the command to send to the phone! I've now unintentionally synced my phone twice in 2 days trying to figure out how to follow the right path.

I am making videos for Instagram and would really like to get a few files back to the phone. (It's where they started!). (Explanation: My computer has a few terabytes of space available and I save most videos there.) I had problems with iCloud and do not want to go there again. The different instructions I've read say that the cable tether should work. I'm on a Windows 10 computer with no Bluetooth.

Please keep in mind I am only moderately savvy and I know SOMEthing will work. Suggestions? Thanks in advance.