The first 2 come with human/elven ancient wonders.. Once you build a bell spire/ crystal lighthouse or a monastery / sanctuary you will get the corresponding building placed on your mountain behind your city.. they are just cosmetic.. The next 2 come from the dwarven ancient wonders once you place mountain halls and bulwark, you will get 2 more placed on your mountain ... The last 2 come from fairy ancient wonders , once you place blooming trader guild and prosperity towers you will get 2 more that are placed on the lake to the top right.. So there is 6 in total that can give you those background buildings