can come from the terrain around your cities (cottages, rivers etc), from trade routes between cities, and the palace will also generate 8
per turn. There are no other sources of commerce. You can see in the city screen how many
each tile and trade route is generating, which is about all the breakdown you need.
If you mouseover the total
icon it'll give a rough breakdown of where your hammers are coming from (terrain, specialists)
For stone the +1
indicates that the tile the stone is on generates one more
each turn it is worked than it would if the stone is not present, so you are right. It does
not mean that the +1
is applied to all your cities, though it will very misleadingly appear in the resource box of all your connected cities. A lot of people seem to make the mistake of thinking that the +1
applies to all the cities, which it doesn't. Stone will however double the production rate of things like the Pyramids and walls in all your cities it is connected to, though it does not directly give any bonus
.