Traders question

Or two questions. Okay, three questions. Actually, can I make it four?

  1. Upgrading a town tier promises to change traders. Which ones? Random ones? Rayya’s caravans? Traders at crafted stalls? All of the above?
  2. How is the random trader’s stock formed? Can I make random traders sell a randomized set of things? (like wood + stone, or different random sets of furniture?)
  3. How can I expand stock for traders at crafted trader stalls?
  4. How does the game choose which random trader to summon? Can we influence it (for example, increase chances for an ore merchant to appear)?

Possible related topic: What is the difference between category and material_tags in entity_data?