As Ian mentioned, once you open the "Stream" folder, you can copy drag the .mts files to your desktop. iMovie might not convert them for editing, and if so you will need to convert them to Mp4 or other format compatible with iMovie in order to edit them in iMovie.
Try converting the .mts files to H.264, Mp4/AAC with the free download, Handbrake. Do that even if the codec already is indicated to be Mp4/AAC. Re-rendering often cures the issue.
You can get Handbrake here:
https://handbrake.fr/
A simple way to do it is to open Handbrake and do a File/Open Source. Navigate to your video and choose it as the source from the resulting screen. Then do File/Start Encoding. Wait a couple of minutes for the conversion to complete. Then save and import the converted clip into iMovie.
-- Rich