How does Robolectric work? Why does my unit test download some jars while first run Unit Test?

I am going to run a Unit Test in Android Studio, to test my method in ViewModel, I use Robolectric to do this.

Everything works correctly. I found when i run test first time, it downloads some jars automaticly. like org.robolectric:android-all:jar:5.0.0_r2-robolectric-1.

So can anybody tell me how does Robolectric download jars, By gradle? or something else? And why does it need these jars?