But on the other hand I don't understand the bottle thing... On the plane, if you lower the bottle then it will be level with the horizon. Can you explain more about this?
You line up the water level with your eye sight.
Lowering it won't make it level with the horizon.
Due to perspective, it will make it tilt up.
You can get it to a point where you can pretend it lines up, but that would just be dishonest photography.
If Earth was flat and the horizon was made due to perspective, you would always be able to draw a line along the horizon, and one along the water level and have them cross at some point.
I find this a poor but better image:
The water is a simple way to get a level reference (which doesn't actually work in a plane if it is manoeuvring at all)
But you can use other references as well.
Based upon how perspective works, you can extrapolate this level line out.
If you have multiple such lines, you can find the point of intersection, the vanishing point.
Which also tells you an angle of elevation of 0 degrees.
Because Earth is round, the horizon is below this.
When close to sea level, it is marginally below, so much so your eyes couldn't tell.
When high up, it gets quite noticable.