TEEJ
Senior Member.
Video description
Looking back on flight tracking software I would suggest that the Qatar Airways flight was QTR211 flying at 35,975 feet.
https://www.radarbox24.com/data/flights/QR211#701234425
The crossing airliner traffic I believe would have been the higher of the two airliners to the north.
TUI Airways/BY2337/TOM41W flying at 34,000 feet.
Playback of airliner identified as "UFO" in the video. QTR211 is aircraft flying right to left in the playback.
https://planefinder.net/flight/TOM41W/time/2018-03-21T16:10#
https://planefinder.net/flight/QTR211/time/2018-03-21T16:10#
https://www.radarbox24.com/data/flights/BY2337#701524702
The airliner following the TUI Airways/BY2337/TOM41W was approximately 4,000 feet lower at 29,975 feet.
A lot of the confusion surrounding this video is the dark coloured contrail. It tends to confuse most viewers who suggest that is either a missile or a military jet fighter.
Contrails depending on the lighting conditions can be varied. This can be seen in the following video. See debunk of that video at following link.
Source: https://vimeo.com/88080046
https://www.metabunk.org/debunked-look-up-org-uks-smokers-video.t3220/
Of note on the "Smokers" video is footage of two examples of crossing airliner traffic similar to what is captured in "UFO/Turkey" video.
See from 06:41 in the "Smokers" video
See from 04:05 in the "Smokers" video. (This airliner is obviously closer/longer contrail)