Paul Kafasis on the Rogue Amoeba blog, on a neat trick provided by Apple’s T2 chip:
On older Macs, the headphone jack and the internal speakers are essentially separate ports on a single output device, and only one of these ports is allowed to be active at a time. Because of this, audio can be sent to either the built-in speakers, or the headphone jack, but not to both. As well, if anything is connected to the headphone jack, the OS shuts off the built-in speaker completely.
With these new Macs, there are actually two distinct output devices. The headphone jack and the internal speakers are separate devices, completely independent from one another.
This makes T2-equipped Macs a lot more flexible for audio professionals, but I still miss the line in jack.