I don't really agree that they are interchangeable. I'm not sure if I can do any better in defining them, but I will try! Sunlight - I would say this is used in the context of distinguishing light from light that comes from other sources. i.e. you can compare sunlight to moonlight, torchlight, lamplight etc. Put simply, it is any light that comes from the sun. You may say that you can see by sunlight, that sunlight causes photos to fade, or that plants use sunlight to create energy, but you wouldn't use sunshine in this context. Sunshine - This refers to the quality and brightness of the light from the sun. So you may have a beautiful day, with no clouds in the sky, and you can say that "you are going outside to enjoy the sunshine", Similarly on a cloudy day you may say there is no sunshine. In both situations there is sunlight, which you are using to see, but would not use "sunlight" to describe the brightness and quality of that light. I hope that makes some sense!

Often they are interchangeable. Sunshine is always direct sunlight, bright and warm in full view of the sun. Sunshine suggests a clear sky with illumination of the whole landscape. The sun shining on an object implies that the object is in direct view of the sun. However, were I to look for a difference, I would say that sunlight is any illumination provided by the sun, whether direct or otherwise. For instance a room with a sunroof is illuminated by sunlight, even on a cloudy day. Sunlight damages paintings over time, even when coming from around a corner. Hope that helps.

IDK

Hmm, that's a tough one because they're essentially the same thing but it would be very easy to use the words in an awkward way. Generally speaking though, "sunlight" is used more often.