It's a bit miss leading, what he means is the portraits use the same skins/hair/accessories that Modern interiors does meaning you can make a character from Modern Interiors and then make that exact same character as a portrait, however to do this it is currently manual, you need to copy all the different parts of the portrait pieces together in some sort of art software like Paint.net or Aseprite.
a tool to automate the process is coming soon tho, I've already finished developing the Portrait Generator which will act similarly to the Character Generator in Modern Interiors, we're just working on the finishing touches right now and it will hopefully be released soon, tho I won't give an exact date.