If you have a Live Photo in your macOS Photos library, and export it using File > Export > Export Unmodified Original, you’ll end up with 2 (or 3) files:
- an image file that is the still image,
- an MOV file that is the video and sound portion of the Live Photo,
- and optionally an XMP file (if you selected Export IPCT as XMP)
Reimporting the image or the movie file will give you separate files in your photos library. If you want to reimport these, make sure they both have the same filename (except for the extension) and drag both the image file and the MOV file (and the XMP file, if you have it) to Photos at the same time. Photos will reimport them as a single Live Photo.