How does BigInteger store its data?

I've been searching around for quite a while, and I've found almost nothing on how BigInteger actually holds its numbers. Are they an array of chars? Something else? And how is data converted to/from BigInteger?

From what I've found, I am assuming that all of arbitrary precision classes, like BigInteger and BigDecimal, hold data as a character array. Is this how it actually works? Or is it just people's guess?

I'm asking because I have been working on my own implementation of something like BigInteger, but I can't figure out how to hold numbers larger than Long.MAX_VALUE (I don't remember the actual number).

Thanks in advance.