>> Do you really mean "physical address" here?

>> External data is created precisely to efficiently access data allocated externally, because it allows direct raw access to a given region of memory (sans bounds checking).

Serach in code "Unsafe." and you understand what means "physical address" as a base and offset.

Eg.

  Unsafe.memorySet(base, offset, 0, size);
  Unsafe.memoryAllocate(size);
  Unsafe.writeInt8(base, offset, value);

https://gist.github.com/mezoni/f85b04bf19dab8333abf