"Modern interfaces" like USB, PCIe, HDMI etc tend to be complex. So forget wrapping your head around those. I think you have a few options: a) Go the FPGA rou

"Modern interfaces" like USB, PCIe, HDMI etc tend to be complex. So forget wrapping your head around those. I think you have a few options: a) Go the FPGA route. FPGA design is not for everyone, and you're working on a black box in a way. But FPGA's capabilities & their architecture tend to be well-documented. And yes there are some supported by open source toolchains. If you can overcome the hurdle of the FPGA design flow, then... your fantasy (and FPGA size) is the limit. Check out OpenCores.org for some ideas on ready-made-building-blocks to use. In this case, you'd want to pick a board where high-speed interfaces you plan to use (like RAM and video output) are on-board. Low-speed / simpler peripherals you can wire up as needed. b) Just pick a reasonably documented SBC (Raspberry Pi, a recent/smallish RISC-V board, or Beagleboard come to mind). Ignore the more complex interfaces (for example, video: start with small RTOS or bare Linux kernel, figure out how to initialize a framebuffer & go from there. No hardware-accelerated 3D shenanigans). Or just use them through OS-provided interfaces to hookup things like a keyboard. c) Forget those modern / complex interfaces, and dive down into the microcontroller world. For example a RP2040 based board + graphical or character LCD hooked up, and you can wire a keyboard from individual switches if desired. :-)