Does not boot, black screen, no caps-lock light

MacBook has recently stopped booting, no liquid or impact damage. The trackpad is clickable however the screen remains black and the caps-lock does not light when tapped some minutes after boot. The CPU heats up. Interestingly when connected to another MacBook (Pro) by USB-C (for example to attempt target disk mode) the trackpad is no longer clickable.

I’ve swapped out the uppercase and display to a spare to see if it’s an issue there along with disconnecting the keyboard but the issue persists which suggests to me it’s an issue with the logic board.

Anyone have experience of this before and what might have gone wrong on the logic board?