We do know, and importantly - can validate/verify, the layers of the air above us - to a certain altitude where hydrogen balloons stop rising.
Above there, all the data comes from the untrustworthy MIC and cannot be verified or validated.
We know that we have positive air pressure, and that we have for a long time. There can't be an infinite sky vacuum above our heads without a barrier in that case. Some suggest that the barrier is a "field" of some kind, and though this is not impossible - we have measured no field in nature of the required strength. Gravity is far too weak (chiefly because it is fictional) to "hold down" the air against its fundamental nature and behavior (basic gas law, and the 2nd law of thermodynamics) - and this is easily validated here on earth where the fictional gravity "field" is supposed strongest.
I'm glad you accept the evidence/data that you can at least wrap your head around because you "know" we have sent scientific instruments up there. But, when it comes to the realms of space and things in orbit doing the measurements, why is that data suddenly becomes untrustworthy? Is it because you suggest that we haven't actually been into space, and so because of that belief (yes, belief - you cannot prove we haven't) you assume all related evidence to be void?
Speaking of void, empty space... You're right, gravity is relatively weak, but all it takes is any amount of gravity for something to be "held down". The weaker the gravity, the thinner the atmosphere, and the lower the pressure gradient. The further you get from the surface, the lower the pressure to the point where there are no longer any air molecules. It's a simple concept. I have to question your understanding of the basic gas law and the 2nd law of thermodynamics though, because you say there CAN'T be an infinite sky vacuum above our heads. Can't be...why not? I've asked this before - what do you think a vacuum is? Do you think it's a suction force, where space acts like one giant vacuum cleaner which would suck the atmosphere away from our surface? The only reason vacuum cleaners on Earth work as they do is because they create a pressure difference between the surface under the vacuum cleaner and the air around it. Same with a drinking straw - you "suck" the air out of the straw, causing the pressure on the inside of the stray to be lower than outside. The positive air pressure pushes down on the surface of the liquid, in turn pushing it up the straw. The very notion of "suction" is a consequence of having gravity and an atmosphere, and is a pushing force.
The vacuum of space is the absence of matter (in simple terms) not because of suction, but because it is empty. On Earth, with the presence of gravity and the absence of anything else (solar winds etc.) no barrier would be needed to keep our atmosphere in place. Remove gravity and suddenly you need an explanation for containment. Like most areas of FET, there are several - four at the current time of writing in the Wiki, none of which can be proven, but at least one discredited.
The closest I can wrap my head around would be the Earth's magnetosphere, but given that reaches out tens of thousands of miles into space, it doesn't quite fit the flat Earth model as the Sun would be contained within the field, not outside it.