how do I put a movie on a thread or is it impossible
You could stuff a movie in a ZIP file and attach the ZIP file as a message attachment. There is a size limit of 1MB for attached files.
A much better way is to put the movie in the gallery or some other online location and link to the movie in the message. Big attachments aren’t a good thing. I try to keep attachments as small as possible (under 100 KB and preferably smaller).
The message attachment feature is really meant for GIFs and JPGs, not movies.