It’s in the post process volume
The SunSky atmosphere uses the Sky Atmosphere system, the main benefit of it is that it is pre-configured out of the box to give realistic results and it has logic in it to handle the sun position given time/date/coordinates which makes it really useful for archviz where you need to be able to simulate the path of the sun in that location.
The main drawback of it is well… it’s realistic. Which limits its flexibility. By default it has a 75,000 lux directional light, which gives you a realistic dynamic range but as I noted previously, it means your auto-exposure transitions can be pretty harsh at times.