“Down” is the same direction as Earth’s gravitational acceleration, toward Earth’s center. “Up” is the opposite, or away from Earth’s center. This applies to all observers on Earth.
Flat-Earthers like to question & ridicule about what would occur on the people “down” there. They just do not understand that to the people on the other side of Earth, “down” is still toward Earth’s center, like it appears to everyone else.
Up and down is relative to the observer. The absolute direction is different because the absolute orientation on Earth is also different. The difference in the absolute direction can be determined from the different celestial objects’ positions seen by different observers.
The human body has a vestibular system that tells us our spatial orientation. We can sense which way is up or down, even when blindfolded and upside down. The vestibular system detects acceleration. If it cannot perceive any acceleration —like in the case of an astronaut orbiting the Earth— we can’t discern which way is up or down. In the ISS, there’s no such thing as up or down.
Flat-Earthers’ ignorance stems from the misconception that down must be toward the same absolute direction everywhere on the Earth. In reality, down is towards the Earth’s center of gravity for everyone on Earth. It is not toward a specific absolute direction.