Sending shadow rays to the sun How is sun modelled for ray tracing rendering? Is it part of the scene, like a
sphere with a centre in some far location or it is just considered as a point?

Also regarding sending shadow rays to the sun, is it, again, sampling a sphere
or just sending shadow rays to one point?
Sun light can be represented as a distant light. Light sources are radial like a bulb, though in the case of the sun, it is so far away from the Earth, that a reasonable approximation is to consider that all rays coming out of the sun are parallel at the Earth's surface. Using a distant light is an approximation. Because it's far away it is also assumed that there is no variation of light intensity as you go up or down (change height).
If you do physically based rendering, the sun is an area light. From the Earth it looks like a small disk so you can approximate this with a large disk that is far far away. Area light like all light sources obeys a light intensity square fall off. So you will need to put your area light far away so that the intensity on the ground or as you change height doesn't vary much. Representing the sun as an area light avoids having to do strange things to the surface shader such as specifying a specular size.