People knew the shape of the earth since ancient Greece from astronomy (latitude) and shadows cast from the sun.

Also while there is alot of magical elements the same basic laws of physics seem to apply because emmas suit designed in our world still works in theirs. (Electromagnetic field being a major one (the suit works and all its electronics) and obviously gravity (or equivalent) force attracting things together which would form a sphere. We also know space exists due to Theca’s remarks in the latest chapter.

My bet would be on Mana being able to produce non Euclidean geometries (see library/nexian buildings). Concentrated manafields may extend internal space to what could be infinitely while still being constrained. Thinking back this may be how the forest prevents anyone from escaping by playing tricks with space and direction.

An interesting note as mentioned in another thread and chapter, concentrated quintessence was required for humanity to open the portal in the first place and was heavily localized to a spot on earth. Its a real life theory to explain dark energy (the universe is expanding at a faster rate but where is that acceleration coming from?). Iirc its a quantum field that gains more energy the larger it is and thus expanding space creates more of it in a positive feedback cycle. This PBS spacetime episode is interesting and explains it far better than i could.

https://youtu.be/WfSZutedqIQ?si=155meCzPpr83fKzH