how to understand list of soundcards; meaning of MID, HDMI, PCH

What's the meaning of MID, PCH, HDMI?

MID is a compound acronym with an unenlightening expansion: it is the Message Signalled Interrupt Capability ID register in the C220 series chipset your machine uses.

(How did they manage to boil that unwieldy term down to MID, you ask? MSI is a well-established acronym in its own right, so this is the "MSI ID" register, or MID. They could just as well have called it the MSIID, or MSICID, but I guess people like TLAs too much. See section 17.1.1.25 in the Intel 8/C220 series PCH datasheet.)

Anyway, MID isn't important here. What matters is that it's tied to the HDMI outputs, which is the true audio output device, from the user's perspective.

HDMI is a digital audio/video transport, most often seen on consumer entertainment equipment like Blu-ray players, but also on some PCs, especially the sort designed to be used as home theater PCs. Since HDMI can carry sound as well as video, it shows up in your list of sound output devices. Some people do use it as a dedicated sound output, sending HDMI to a DAC while video goes out another path, such as VGA.

I don't know why you see three HDMI output paths, but it is almost certainly not because your computer has 3 HDMI output connectors, or even the capability of such. I hesitate to even guess what the distinction among them is, though if you post your lspci -v output, I could research the ICs involved and would probably figure it out.

PCH is what you'd call your actual "sound card," though it isn't actually a card in the machine. PCH is Intel's name for a family of ICs that include sound output. Because there have been many generations of PCH, you often see it followed by a fragment of the IC part number, like PCH C220, which helps you decide whether a given driver is compatible with the particular PCH family variant on your motherboard.

Which one is connected to the moss-green standard plug?

PCH.