The Over:Board is a carrier board for the Raspberry Pi Compute Module 4 that turns the compact machine into a mini-ITX motherboard. The Over:Board features a host of I/O, including dual HDMI and USB 2.0 ports. There is a PCI-Express slot too, along with a CPU fan header.
https://www.notebookcheck.net/Raspberry-Pi-Bring-the-mini-ITX-form-factor-to-the-Compute-Module-4-with-the-Over-Board.514583.0.html