Why the Milky Way appears to be arching across the sky? The Milky Way galaxy is disc-shaped and earth is in its plane:
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In the following image the milky way appears to be arcing across the sky:
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This looks like an out of plane view of the milky way... How is it possible? Why an arch and not a thick straight line?
The Earth is a sphere (or is nearly a sphere). So to make a map of the whole Earth you would need to project it onto a flat surface. When you do this you create distortions. For example, on many maps, the straight line between New York and Japan looks like a long curve. It is not actually a curve, but when you stretch the surface of Earth to make it flat, some straight lines become curved.

The sky is also a sphere. If you ignore distance and just think of the direction of each point in the sky it would be a huge sphere that surrounds you. When we take a picture of the sky we must project that sphere onto a flat surface. That causes distortions, and it means that what is a straight line on the sphere of the sky is distorted by the projection into a curve.

If you choose a projection in which the milky way goes straight across the middle, you can see that the milky way goes straight across the sky.

The appearance of an "arch" is a distortion caused by stretching the image to fit on your flat screen. It is not real.