No clue on the specific numbers as to how much pop = how much gilder. But From my last long game it seems that overall pop was the base for the tax generted. Also saw new food options/tech/builds to increase food production which gave me a surplus of food by turn 140 with a modest gilder income of 120 per turn after expenses.

You get 0,1 Gildar from every citizen per turn. More pop = more tax. But you can not raise or lower taxes. But as you wrote, you can raise your tax income by improvements.

Mana: There is a talent that gives you +2 mana per turn (attunement). And you can increase the mana income by shards with researching bigger shrines (magic - techtree).

They changed it so that normal Sovs produce 2 mana per turn.  It used to be Sovs - 3 mana per turn and Shards 1 mana per turn, and then in one of the beta patches they changed it to 2 (Sov Base) and 2 (Shard Basic).  Sovs with Attunement do get an extra 2 per turn, and Greater Shrines produce an extra 1 per turn per Greater Shrine.

Best regards,
Steven.

I'm still not sure I understand, is it for each citizen created in the city, or each citizen used in the city?

Correct me if I'm wrong, but since the population is a global resource, it is possible to have a very populated city without many buildings, and a small city with more buildings than citizens.

In other words, if I have a city dedicated for gold, should I invest in its size or just build gold-giving buildings and gain the population from a different city?

(and yes, I know I should also improve the city to get the gold % bonus, this question is only to clarify my confusion)

You get for every citizen 0,1 Gildar. Not only the workers have to pay there taxes.

Thats correct.

For me...that is something that goes hand in hand. In small citys only a merchant is possible. So i need more citizens to lvl. up. my city, so i can build markets, guilds etc.