How did Einstein develop General Relativity? What does it mean for different kinds of masses to be equivalent? How does gravity do what it does?

How did Einstein develop General Relativity? What does it mean for different kinds of masses to be equivalent? How does gravity do what it does?
The Milky Way is getting largeerrrrrr. Wait… how com? Check it out. And more about light in the void.
What makes Iceland so warm despite its high latitude? What powers everything from glaciers to geysers?