The same way it currently does, but the sun is significantly closer and smaller, also is has a range of light, like a lamp shade that makes sure the sun isn’t always visible even though it’s always above the disc earth, as is the moon (and don’t even start trying to reason with the moons movements on a flat earth model, or explain either types of eclipse… It only gets worse the deeper you look)
For the oblivious folks who genuinely believe in the flat earth, the issue they mainly have a complete lack of understanding of scale. Having a sun that has directional light naturally, or that light has a limited range it can travel unimpeded, are symptoms of it.
They cannot fathom the size of our planet, some flat out deny the southern portion of the earth is even real. They have never traveled in any meaningful way to understand the distances they likely travel on a daily basis, let alone larger distances. Expecting them to not attempt to point at daily items like a desk light and think ‘that’s how the sun do’ is a massive failing on general education more than it is those folks being willingly ignorant.