How is .Random.seed initially created?

I'm aware that it's possible to set the seed by using set.seed, but restarting an R session gives a new seed every time. According the R Documentation:

Initially, there is no seed; a new one is created from the current time and the process ID when one is required. Hence different sessions will give different simulation results, by default.

I looked through the R Internals guide, but it only describes what .Random.seed is, rather than how it is initially created.

How does R create the seed at the beginning of a session?