I can not trade with other civilizations because they are too far away. I am on the map where there is the North Pole at the middle. I could send a trade route to one city but no trade post was created. Then I could not reach other cities. Can not understand trade post.
Trade posts are only created, if the trader completes it's route. If the trader gets pillaged, the destination or origin city gets captured or loyalty-flips, the trade route gets cancelled. Reachable distances are 15 tiles on land and 30 on water, iirc, from the nearest trade post.
It reads for gold that exploited tiles are 23 but how ? I have few buildings on luxury resources that is it.
You're probably getting 18 gold from specialists working in the Commercial Hub, then 4 come from worked spices and tea and the last 1 from the worked lake tile, that add up to 23.
Specialists working in the districts are counted as working a tile, for some reason. Usually CH specialists yield 4 gold, but if CH is fully developed, that is to say, it has all possible buildings built, then every specialist yields 6 gold.
As you have a lot of citizens in the city, they got automatically assigned to work, and as specialists too. You can check where your citizens work in the Citizen Management tab - select a city and click on the head icon. Worked tiles will display a white head.
In smaller cities you can micromanage and lock them to work tiles you need the most. You can also set yield preferences by clicking on those small circles next to respective yields, just above the city name banner, bottom right, then your citizens will be automatically assigned to work the tiles giving those yields first.