SideMiniHeadless
HDMI dummy plugs: what they do, and when you do not need one
A dummy plug is a ten-dollar connector that tells the Mac a monitor is attached when none is. For a Mac with no display it is the cheapest way to make Screen Sharing usable. It also creates two problems people do not expect, and there is now a way to skip it entirely.
What it actually does
The plug carries a small chip holding the identification data a monitor would send. The Mac reads it, believes a display is connected, allocates a real framebuffer and renders a desktop into it. Screen Sharing then has something to show you.
It sends nothing anywhere. Nothing is displayed. It exists purely to make the Mac behave as though a screen were there.
Which one to buy
Match the port: HDMI, DisplayPort or USB-C depending on the Mac. Buy one that advertises the resolution you want to work at, commonly 1920×1080 or 3840×2160, because the plug decides your remote desktop size and you cannot change it afterwards.
The 4K versions cost a little more and give a much more usable remote desktop. Cheap plugs sometimes advertise refresh rates the Mac dislikes, which shows up as flicker in the remote session.
The two problems nobody mentions
It occupies a port permanently, which on a Mac mini with three or four ports is a real cost.
Software now thinks a monitor is attached, because that is the whole point. Anything that behaves differently with and without a display, including automation that connects an iPad only when no monitor is present, will make the wrong decision. Tools that handle this let you mark a specific display as not counting; SideMini has that setting under Advanced → Monitors.
The software alternative
A virtual display does the same job with no hardware: macOS gets a real framebuffer, Screen Sharing looks normal, and the size is a setting rather than a purchase. It also disappears when you plug a real monitor in, which a physical plug obviously cannot do.
This is one of the things SideMini creates automatically, in 1080p, 1440p or 4K, on any Mac with no monitor attached. If you already own a plug, keep using it and mark it as not counting.
Do you need one at all?
Only if you look at the Mac remotely. A machine you administer entirely over SSH does not care whether a display exists, and neither do most background services. The blank desktop only matters when you want to see the desktop.