Its all handled "Behind the Sceens" as far as I can tell. Not noticed any increase in Tourism profits when adding trade routes. But then the Tourism calculation is not very helpfull in game, like it tells you nothing other than X creds form tourism. Disapointing tbh. It should have a run down of what effects it tbh.

Tourism is very definitely not a constant 3.7 bc per planet before modifiers; a custom faction with the Wealthy ability (unlocks tourism from the beginning) and no tourism modifiers had a tourism income of about 1.8 bc per planet on the homeworld and first colony on the second turn, which dropped down to about 1 bc during the colonization rush and has slowly been growing. I suspect, though I don't know, that the base tourism income is something like the average population per planet multiplied by the GalacticTourismSpendingMod defined in GalCiv3GlobalDefs.XML.