It's a sort of optical illusion and we're all taking part but none of us knows it.
You look up you see sky, you look down you see ground. The ground looks basically flat, with some hills and valleys, lakes etc as far as the eyes can see. Everyone sees these same things because the world is so huge.
I can totally understand how people used to think the world was flat and not realise it's spherical. They were going by what they could see and feel. If you were in a space ship and moved far enough away from earth, then you can see it looks spherical. If you could see people on it, all their feet would be on the floor but some of them would look upside down. If you turned your spaceship around, the upside down ones would look the right way up and those who had been the right way up would now look upside down.
You think Australians are upside down because you live in the UK on the opposite side of the world and your life naturally revolves around you so that's your starting point. In your mind you're the one who is the "right way up" and others are "upside down".
Your starting point isn't real though, it's just your perception of reality. In actual reality, the universe doesn't centre around you. Nobody is any more "the right way up" or "upside down" than anybody else. We're all the same amount of "right way up", but that's not how you perceive it because we're all in an optical illusion that the bit of earth we stand on is "the right way up". Combine that with the perception of reality that centers ourselves and everyone thinks it's others who are "the wrong way up".
Geography isn't my strong point, but I think if you look at a globe the UK is somewhere on the side. I think a globe has the earth held at the angle it's spinning. So if you're picturing a spherical earth you're not the right way up or upside down, you're actually sideways on @XMissPlacedX 😝
Whoever wanted to know why sky is blue and space is black. Space is a vacuum and black isn't a colour, it's an absence of colour. White isn't a colour either, it's all the colours together at once. Colour is what you see when sunlight hits solid objects like atoms (atoms make up everything, even you are made of atoms) and bounces off and spreads out. The sky is blue because it's not a vacuum like space, it exists within the earth's atmosphere. The atmosphere is a layer of gases surrounding the earth, including oxygen that we breathe. These gases are made of atoms too. So sunlight bounces off everything and you see colours.
You can buy clear crystals to hang in your window. When the sunlight hits them some of the light goes through the crystal because it's clear and because it's a very solid object with a lot of atoms in it, the light gets separated out at certain angles so you see a rainbow coming from the crystal and landing on your wall.