Possible to boot Linux on MacBook Pro in screen mirroring mode vs dual screen? Looking for a “bios” toggle to tell HDMI output to mirror display vs split screen (at hardware-level vs OS-level). Not sure this exists…
It does not.
The BIOS screens that you see on modern PCs, besides not being BIOS (rather UEFI) are just not available to modern Macs. That's what you get when you build the software AND the hardware. You don't need a separate interface to adjust the hardware, it is all done from inside macOS.
That said, it is possible to make some changes that take effect pre-boot, using the nvram command
nvram -p
Should give you all the settings currently set in non-volitale RAM. Many of which you can set yourself. A quick search should turn up resources that show the possibilities.
Though I doubt you can do what you want to do there. Those settings are controlled exclusively by macOS, so you would likely have to set them inside the OS of anything you booted the Mac from.