HDMI2C allows network control (via WiFi) of monitors connected to it via HDMI (via DDC). This allows you to build your own tooling and control systems for your monitors, rather than relying on awkward inconvient buttons and OSDs. Think of it as an API for your monitors.
This repository contains the hardware design, made with KiCAD. You can see the generated BOM and schematic here.
It's floating around somewhere, I'll tidy it up and upload it at some point.
Not all monitors implement DDC that well. Particularly of annoyance, some models I tested would not listen to DDC commands on inputs that were not active, which, given this project relies on you having a spare HDMI input which never carries any video, is kind of a deal breaker.
You can achieve the same thing without having to build your own hardware with a Raspberry Pi 4. Support for dual monitor I2C (DDC) was added shortly before I started this project - guess I didn't check enough :-). This is a much more accessible means to achieve the same goal. However, if you want a more minimal solution that doesn't require an entire embedded linux system, this project may be for you.