For anyone interested, I was watching this guy named Alexey Chekurkov for a few years now and I think he has a genuine antigravity device. This is a third-party video with some English translation:
And the original version of one of the videos you saw above is this one:
His YouTube channel is here:
The videos are in Russian language but you can use the CC (subtitles) feature of YouTube for a (approximate) translation to English.
He studied Viktor Grebennikov's work and seems he found the key of his antigravity technology.
On his channel you'll see old videos explaining Grebennikov's work and also presenting details of the device including electronic schemas.
The only thing he seems to keep secret is a metal alloy he discovered (or made), it's using that alloy in a disc plate located in the middle of the device.
Of course I can be 100% sure only if I can personally check the device and see it flying but from what I have seen in these years I think the device is genuine. If it's real then this is the technology used by UFOs when flying in our atmosphere.