I have never actually merged a mod and I dont know if next war uses python or not, so I cant help there.
For refrence, XML doesnt trigger anything. It just gives data to Python and the .dll. The techs 'enable' the unit. Triggering generaly refers to a dynamic change like events.
That said, techs unabling units are defined under prereq tech in unitinfos. All that Techinfos does is give info such as name, a few abilities, and prereqs for other techs.
For techs enabling another tech, that is done in the part <PrereqAnd> and >PrereqOr> IIRC, prereqor is done for linear tech advancement. Prereq And is done for tech needs both. Or is also used for either or in techs.
NOTE: In prereqs, it defines what is needed for that tech, not the next one.
Remember, when in doubt, poke around in the XML. Most things that people ask about Firaxis already did. You just need to copy and paste.