Sadly lacking in details, this developer, "Nyan Satan" (his YouTube channel name) convinced an Apple $50 Lightning to HDMI adapter cable to run Doom! But at least he made a video demo of it running.
The adaptation of the Lightning protocol to the HDMI protocol requires more than a bit of work. The cable contains a microprocessor with a very scaled-down version of Apple's iPhone operating system, iOS. And Nyan got into the CPU and convinced it to dance to his tune!
Now, here's the interesting bit: that cable adapter's CPU has more processing power than most of the PC's that we were running back in the '90s when Doom released!
https://it.slashdot.org/story/25/02/05/1314221/developer-runs-doom-on-50-apple-lightning-to-hdmi-adapter
The adaptation of the Lightning protocol to the HDMI protocol requires more than a bit of work. The cable contains a microprocessor with a very scaled-down version of Apple's iPhone operating system, iOS. And Nyan got into the CPU and convinced it to dance to his tune!
Now, here's the interesting bit: that cable adapter's CPU has more processing power than most of the PC's that we were running back in the '90s when Doom released!
https://it.slashdot.org/story/25/02/05/1314221/developer-runs-doom-on-50-apple-lightning-to-hdmi-adapter