Epistemic status: Big if true/I am clearly an idiot for even posting this.
Some apparently real journalists have been approached by (& approached) several intelligence officials, some tasked specifically with investigating UFOs, who claim that the DoD has had evidence of alien intervention for a while in the form of partial & mostly-whole fragments of alien aircraft. A followup article where the publication outlines how the editors verified this persons' and others' claims and affiliations is here, and a part 2 is expected tomorrow.
For some reason - very possibly because it's complete nonsense, or because they haven't had time to independently verify - the story has only been picked up by NYMag so far. The consensus among the people I've been reviewing this article with, is that it's either a complete hoax (i.e., the entire thing nearly top to bottom is some deliberate deception) or there's a non-negligible (>5%) chance aliens are here. I would love for someone who has a good understanding of the material to give an explanation (including possibly on priors, just thinking clearly about the content of the article) of why my friend group should discount this out of hand.
Thus far I have been unconvinced by most stories of why we should to-the-point-of-not-caring-about-UFO-sightings-expect Aliens have to be big and obvious and tile the universe with fun, as opposed to operating some sort of noninterventionist monitored lightcone.
The military released some videos but there's plenty of reporting that it didn't released their best-quality videos precisely for the reasons you listed. Having the best quality videos that are taking by fighter jets would reveal information about the fighter jets that the US military does not want out on the open so those high-quality videos are classified in a way the videos we have aren't.
The kind of people who are in the know and who are qualified to interpret those videos are military personnel. People like David Charles Grusch.
There's no good reason why someone like David Grusch should falsely tell Congress under oath that the military has intact vehicles “of exotic origin (non-human intelligence, whether extraterrestrial or unknown origin) based on the vehicle morphologies and material science testing and the possession of unique atomic arrangements and radiological signatures”.
Lying to congress that way would be a way to confuse the Chinese but it results people in Congress trying to engage with secret military programs in a way that the military doesn't like. While some lies to Congress don't get prosecuted, incorrectly telling Congress that the US military has intact vehicles of exotic origin seems like the kind of lie that Congressmen would want to see punished. It seems to me like a very strange conspiracy theory that the US military would purposefully mislead congress in this way.