What do flat Earthers think causes the "magical downward force"?

They don’t know - and not all of them agree that there is a “magical downward force”.

1. Gravity is real - but the Earth’s disk is very thick so the force is very nearly vertical.

I have heard them say:

Can we trust the Bible when Constantine and the First Council of Nicaea took out many books of the Bible and altered existing translation by removing things?

If (1) was the case - the center of gravity of the Earth would have to be so deep down to avoid gravity pointing off at an angle when moving towards the “rim” of the flat earth that the difference in height of a mountain would be negligible and we would get very, very nearly the same gravity everywhere.

4. Gravity doesn’t exist - things are held down by air pressure.

…and there are probably other ideas out there.

Alex Cora Takes Blame for Red Sox Woes After Extra Innings Loss - Sports Illustrated

If (3) was the case - you’d have the same problems as (1) - but this weir...

The trouble with (1), (2) and (3) is that these cannot explain why the force of gravity varies with height above sea level…less at the tops of mountains - more in valleys.

2. Gravity does not exist - the Earth is accelerating upwards at 9.8 m/s2 which seems like gravity.

Tesla’s head of Optimus humanoid robot leaves the ‘$25 trillion’ product behind - Electrek

If (2) was the case - then the acceleration would have to be EXACTLY the same everywhere - and it’s not.

3. Gravity doesn’t exist - it’s a form of magnetism that works on non-metals.