From what I understand, this is possible by playing and taking apart actual units and recreating the game logic somehow? Sometimes, like in the case of the Splatterhouse game I contributed to, videos of the game being played are used if the unit's too obscure or hard to come by.
Though take what I say with a grain of salt, as I'm by no means a developer; though it's worth nothing people have been doing these since the late 90's, if Madrigal's emulators are to be believed.