Its really not complicated at all, if you understand what all the core concepts are in the first place. Pretty much every game concept has its own section you can research.
You research from 3 trees. The top is divided into the three weapon types, the bottom is the three defense types, and the middle is ground combat. Thats it.
Tourism, diplomacy, influence. they aren't any more complicated then this as far as the research tree is concerned. Tourism is a part of the economy tree. Just like the military example, it has three parts. Tourism, trade, and raw income. Diplomacy is in itself a tree, which is divided into three parts: government, relations, and influence. If your curious, government provides empire-wide passive bonuses, relations increase your standing with other empires and give you more diplomatic options, and influence increases the rate your territory grows.
All the tech trees follow this cookie cutter formula as far as I could tell. At first these trees can open up new buildings, components, and strategic options, but as you go down these trees they mostly just provide upgrades to your existing stuff, or increase bonuses you already have, or give you the opportunity to spit out another 50 supply ships to get that next 10% production bonus.