How to copy an iMovie from one MacBook Pro to another MacBook Pro

Are you referring to copying a movie that has been exported from iMovie, or are you referring to copying the iMovie project and/or the library itself?

Air Drop would work fine to send a copy of an exported movie. 

The iMovie project itself cannot exist outside of an iMovie library.  So you would need to do an Edit/Copy of the library icon itself and then Edit/Paste the library into an external drive as follows:  To transfer the library from one Mac to another, Edit/Copy the library and then Edit/Paste it to an external drive that is formatted either Mac OS Extended (Journaled) or APFS.   Then physically plug in the external drive to your destination computer. You can then Edit/Copy, Edit/Paste, the library from the external drive to your destination computer.  After confirming that the transferred library is working, you can delete the copy from the external transfer drive if you wish.  The destination Mac must be running the same version of iMovie that was used to create the movie.

I am not aware of any other way, but if anyone on the forum knows of one, hopefully they will chime in here.

 -- Rich 

While possible to use Airdrop, it would be slow and potentially unreliable for a large iMovie file.

File sharing would be better. But IMHO it may just be easiest to use a flash drive to copy/transfer your iMovie Library or exported video from one MBP to another.