What are the internal workings of Google Voice?

No numbers are inherently "SIP" numbers or any other type of numbers, they are just numbers. Numbers are handed out to VoIP providers and PSTN line providers alike from number vendors (such as 360 or Level 3, they do not generate the numbers themselves - though they can, however this doesn't mean they own the number, it will route only within their network if it is not routed through a number vendor). These number vendors control the routing of the numbers between other number vendors and the providers they provide backbone service for. This is typically where the transfer between PSTN and VoIP takes place, on something called an IP Backbone.

SIP is just one of many VoIP protocols. Google has its own SIP servers to handle the signaling for its service - its clients will need to sync with this server and signaling that creates a call is handled this way. These servers are called Session Border Controllers and are often accompanied by applications servers (for hosted voice, but integrated systems don't always require application servers). Their SBCs communicate with the servers of their number vendor in order to have the call routed properly (unless it is an in-network call, then it will never leave the VoIP provider's network).

Hopefully this image will help