Background: In Civ6, a Trader can only pass between land and sea using a City Center or Harbor district. Also in Civ6, a civilization can establish a Trading Post in a city with which they have previously completed a trading route.
Question: Can a Trader use a City Center or Harbor in a foreign city to pass between land and sea, and if so does it make a difference whether the Trader's civilization also has a Trading Post in that city?
I would be surprised if the answer to the first part of the question isn't yes. On the other hand, since districts are generally owned and operated by one's own civilization, I suppose it's possible that other civs can't use them for this purpose.
Bonus points for clarifying a related vague issue on ranges: According to Civopedia, ranges are not "carried over" when transferring between land and sea. Ranges are 30 over sea, and 15 by land. If a city is 25 sea tiles and 10 land tiles away from another city, does this mean it is within range? In other words does the Civopedia language mean that these are two mutually exclusive allowances?
Best Answer
Yes, a Trader can use a foreign Harbour (or a variation thereof) district or a City Centre to transition between sea and land.
However, Trading Posts have nothing to do with this action. Trading posts are not improvements or buildings that civilizations buy, they are established on a per city and per civilization basis. That is, you can have multiple trading posts (one per civilization) in your city.
They are established once a trade route, foreign or domestic, to a city has come to term. They do not cost upkeep but provide bonuses (typically gold) depending on your civilization and civics. They also replenish movement if you move through a city which features one of your trading posts.