The barrier repo is basically dead, input-leap is a fork from two of the previous maintainers and still actively developed: https://github.com/input-leap/input-leap/issues/1414 So unless you’re using an apt based distro (which don’t seem to have it packaged yet), I’d recommend using that instead.
You also need a powered adapter for HDMI 2.1 in this case. The passive adapters work, because the DP output on the computer usually supports switching to HDMI output. But for that to work, the driver must support it, so it has the same problem as the HDMI port (which supports 2.1 on a hardware level, but not with AMDs open source drivers)
If you want 120Hz, only active adapters will work, which cost around 40$. If you don’t, the included HDMI 2.0 will be enough
If you don’t need 120Hz, you don’t need HDMI 2.1. You can get 4k @ 60Hz with the HDMI 2.0 that the Steam Machine has, so you can use just any TV.
Passive DP-to-HDMI adapters only work if the device on the Displayport end supports Dual-Mode, i.e. using the Displayport to send an HDMI signal. They often do, but it would require the same driver support for HDMI 2.1. So this would require an active adapter.
ARM boards with slotted RAM use the same type as x86 (although mostly LPDDR, as found in laptops), so I assume there isn’t any difference that is related to the CPU architecture.
Except the circumstances in which they run on gasoline are not that exceptional: https://theicct.org/publication/real-world-phev-use-jun22/ And the lower weight of the battery is partially offset by needing an additional drive train, a transmission with multiple gears (purely electric cars usually have fixed transmission) and other smaller parts required for the combustion engine. These also increase the required maintenance a lot (e.g. requiring oil changes).
It runs as part of steam, so you would need to add the program you want to use to steam as an external program and start it through steam. Steam Input then runs as a translation layer around the other program. At least that’s how it works on the Steamdeck.