Until I got too old and started getting overly skittish on the trails, and more and more scared of falling, which was becoming more likely with age, I used to love hiking in the mountains. I was never a climber or a mountaineer, but I loved the mountains and had enough stamina at the start of each hiking season for 1,500 to 2,000 feet of vertical elevation gain, and by the end of the season I was able to do up to 5,500 feet of elevation gain in a day, which I generally did on at least one hike near the end of each season. The summits I climbed to were generally from eight to nine thousand feet above sea level, and surrounded by higher mountains that would have required climbing and mountaineering skills to ascend.
(Hiking is when you're just walking. Anybody can do it. Climbing and mountaineering are technical skills that must be learned. I was just a hiker.)
But one time I spent two or three weeks hiking around Flagstaff, Arizona, and one day we hiked to the top of Humphrey's Peak, which is 12,633 feet above sea level. The trailhead is around 3,500 feel below the summit, so we started hiking at a higher elevation than most of the summits I was accustomed to. Somewhere around 11,500 or 12,000 feet I could feel that it was a little bit harder, due to the lower air pressure and resultant reduced oxygen.
Rounding off the numbers, there's 68% as much oxygen at 10,000 feet as there is at sea level. At 12,000 feet there's 63% as much oxygen. And at the top of Humphrey's Peak there's 62% as much, which explains why I was feeling that climbing was starting to get harder.
And so I get to the point of this post: Humphrey's Peak is the highest point in Arizona, so when you are up there you are higher than anything within eyesight. And from that height, the horizon is very clearly curved. You can hold your hiking pole out in front of you, against the horizon, and see the horizon bend down from it at both ends. When you look at the world from the vantage point of Humphrey's Peak, you can SEE the curvature of the world.
The only other time I've been on the highest point within view was on top of Ben Nevis, in Scotland. Ben Nevis is about 4,400 feet above sea level, and the parking lot where you start your hike is around 100 feet, so you climb 4,300 vertical feet along a wide stone path that's very easy to climb, but can make the bottoms of your feet sore by the end of the hike. From the top of Ben Nevis the curvature of the Earth is not easily apparent to the naked eye.
I recommend that any flat-earther who is reasonably fit (it is a strenuous hike) and can make it to Flagstaff, climb Humphrey's Peak and see for yourself. It's a beautiful hike. If you are not an experienced hiker, bring a friend who is, or hire a guide. Nature is beautiful, but she can be unforgiving to those who are unprepared.
BTW, the reason there's less oxygen high up in the atmosphere is because there's less air pressure, so there's less of everything. And the reason there's less air pressure is because the weight of overlying air, pulled down by gravity, presses downward and compresses the air below. The lower you are, the more air there is above you being pulled down by gravity.
Something similar happens with water, except that water is not compressible so the density does not increase, but the pressure does, which is why an inflated balloon will get smaller if you force it under water, and why a badly-constructed submarine will implode if it goes too deep.