How do people calculate pi to the hundredth+ decimal place?

So I know that pi is the ratio of a circle's circumference to its diameter (and the ratio of r³x4/3 to the volume of a sphere).

Apparently even the circumference of the universe needs less than 40 decimal places to be more accurate than we would ever need to worry about.

So my question is, how do we determine the decimal points beyond this? If pi is a ratio and even the largest conceived circle only gets you to ±36 places, how do we determine what the subsequent numbers are?