An FPGA is essentially a reprogrammable computer chip, or integrated circuit (IC), that can behave as another computer chip. It is widely used in the development of new ICs.

The MiSTer FPGA project uses an off-the-shelf Altera DE10-nano development board, which has a combo FPGA + ARM SoC on it. The OS, USB controller input, and some other stuff runs on the ARM core, and the FPGA is reprogrammed upon launching a core to behave as closely as possible to the original hardware that it’s emulating.

FPGAs can either be pre-programmed or programmed on-the-fly. In consumer hardware, FPGAs and CPLDs (essentially weak FPGAs) are used when you need an IC produced in small scale, or when you need to be able to change the functionality of the IC with updates.

People know so much because they take the time to learn, and it does take a lot of time and patience.