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The short version is ”gravity”

The longer version is the concept of spherical geometry and antipodes

Given a massive enough body (like the earth), at any given point on the surface will feel ”up” because ”down” is relative to the direction gravity pulls

An antipode is the ”same” position on a spherical object as yourself, but with the coordinates reversed (eg the position 100E 50N has it's antipode at 100W 50S – literally the opposite side of the planet)

The only reason you feel north mist be ”up” is because most maps are published with a northern-hemisphere-biased orientation (due to the fact that most of earth’s population lives in the northern hemisphere (indeed, most of earth’s habitable land surface area is in the northern hemisphere (along with most of the landmass at all)))

the southern hemisphere could just as easily be considered ”up” – there's no physical, geometric, geographic, or mathematical reason for it not to be: except convention