Question

Q/ Does rainbow has an end or starting point, and why rainbow has an arc shape and never a square?

Answer 1

A rainbow is actually an arc - technically, a segment of a full circle.

Unfortunately, a rainbow is created at a very specific angle away from the Sun (or whatever light source is being used).

In other words - assuming the rain and sun continue - as you approach where the apparent end of the rainbow was when you started, as you move the rainbow moves too.
The rainbow is an arc because the horizon stops the rain which is producing it… as the rain hits the ground.

It’s easy to show this by simply going out on a sunny day and spraying a mist of water and looking at it with the sun at your back.

The rainbow is curved because the set of all the raindrops that have the right angle between the observer, the drop, and the sun, lie on a cone pointing at the sun with the observer at the tip. The base of the cone forms a circle at an angle of 40–42° to the line between the observer's head and their shadow but 50% or more of the circle is below the horizon, unless the observer is sufficiently far above the earth's surface to see it all, for example in an aeroplane.