I’ve read books (mainly Star Wars) that tend to have a species language (e.g. cheunh for chiss) and then trade languages that the greater part of a region also speaks (e.g. Minniesiat and Sy Binsti). That made me curious as to how trade languages form.
There are a few different uses of the term "trade language".
One is a language that lots of different groups learn as an L2 (i.e. not as their native language) because it's useful for trade. For example, during the Roman era, people all across Europe and the surrounding areas had some proficiency in Latin; people in North Africa might speak Punic as their native language, and people in Iberia might speak Iberian, but they would both know enough Latin to do business with each other. Or, think about the role of English in the modern world.
These sorts of trade languages tend to come about when one group has enough social or political power that people across a wide area consider their language valuable to learn. I don't know much about Star Wars, but I can easily imagine the language of a Galactic Empire becoming one of these.
Another meaning of the term is a language variety that arises specifically for trade. In this meaning, it's something like a creole, with features of different languages combining together as people try to figure out a way to communicate across language barriers (and then pass the language down to their children). The "lingua franca" used across the Mediterranean during the Renaissance, for example, was probably one of these; people speaking Romance, Greek, Arabic, and Turkish ended up adopting words and pieces of grammar from the other languages in order to do business and ended up with an amalgam of all four. Or for another example, Arabic-speaking traders wanted to do business with Bantu-speakers along the east coast of Africa, and the end result was Swahili.
For this sort of language to arise, you need two or more groups of people with a strong need to communicate with each other, and without the sort of power differential that leads to one group's language being adopted as a default.
It's also worth noting that these two meanings aren't mutually exclusive: they're more like different points on a spectrum. When languages come into contact they always have an influence on each other; the question is how strong that influence is, and how intelligible the end result is to someone who only speaks one of the parent languages.