But how to get movie from the dow load folder to the iMovie.?
Sorry, David. Could you just clarify for me? You used AirDrop and it worked. Got that. Are you saying that you used AirDrop to move the video from the iPhone to your Downloads Folder?
If I've got that right, then you would almost certainly want to put that in your Photos app along with all the pics.
To do that, Click/Drag the File from Downloads on to the Photos app icon in the Dock. (If you don't keep the Photos app in the Dock, then open Photos and the icon will appear in the Dock).
Or you can open Photos, then Click on File > Import and navigate to Downloads and click on the video file. More hassle that way.
I'm advising the Photos app rather than importing the video into the iMovie app because it keeps all your pics & videos in the same place for easy viewing.
But if you need to Edit the video and need the sophistication of iMovie app, then you can open iMovie and File Import from Downloads.
Ian