What do you mean by this? The more gold I make per turn, the more research I make?
You have a common terminology problem:
is commerce, not gold. Gold is
. Unless maybe you are still using plain Civilization without expansion packs - I think it was originally the same symbol, which was very confusing to people. (It is made even more confusing by the existence of the gold resource, which is something else entirely - although working a tile that has the gold resource and a mine on it also provides a lot of commerce.)
Commerce, not gold, is what a city's worked tiles directly provide to you (in addition to food and production), and it also comes from trade routes (and possibly a few other things).
Commerce is converted to gold, research, culture, and/or espionage points depending on how your sliders are set. Each of these things can also come directly from other sources, like specialists or buildings (especially for culture and espionage - buildings are usually the main source of these).
Working more tiles that provide commerce, or tiles with higher commerce values, gets you more commerce. Having more, or better, trade routes also gets you more commerce. There are also some buildings that increase the amount of commerce you get, for example a harbor gives +50% commerce to each trade route that city has.
The buildings in each city only modify the commerce, gold, research, etc. that that specific city produces. A monastery gives +10% research to the city where it is built. Two monasteries in the same city give +20%. As with pretty much ever other case, the percentages add before applying them - they are not multiplicative. If you have a city produces 20 RP/turn and you build 2 monasteries and a library (+25% research) you get 20*(1+0.1+0.1+0.25) = 20*1.45 = 29 research points from that city, not 20*1.1*1.1*1.25 = 30.25.
(Side note: the +50% per trade route from a harbor is not very effective if all you have is domestic trade routes since most of them give 1 per turn and the fraction is rounded back down to 1, unless you are playing a mod that allows fractional trade routes and only rounds after adding them all together for the city. It is better if you have foreign trade routes since those usually give more, which is a good reason to have open borders with some of the other civs. That first open borders agreement with a civ you can reach via roads, rivers, and/or coast without intervening civilizations or barbarians blocking them can more than double your trade route commerce, especially if you have a few harbors.)