The answer is that the UFOs are, of course, fake, but so is everything else. Every part of this video — the car, the scenery, the clouds — is CGI. Wired explains:
"UFO Over Santa Clarita" was a painstakingly crafted joke played by Aristomenis "Meni" Tsirbas, the director of the 2007 computer-animated film Battle for Terra who has also contributed visual effects and animation work to movies like Titanic and Hellboy and several Star Trek television series. A long-time champion of "photorealistic" CGI, Tsirbas and his team spent about four months mimicking the look of an accidental extraterrestrial encounter captured on a smartphone. And until now, Tsirbas hadn't revealed the truth to anyone outside a handful of friends.
"The point of the video was to prove that CGI can look natural and convincing," Tsirbas told Wired. "Everybody assumes the background and car are real, and that the UFOs are probably fake, especially the over-the-top mothership at the end. The general reaction is disbelief, so I usually have to prove it by showing a wireframe of the entire shot to prove that nothing is real."