Typically they have a root to them. There are words that don't and are made up, like yeet (which I'll consider a word because of its usage and common knowledge), but other words like "microscope" are are derived from Latin or something else. The example here is from microscopium. There's a lineage and things modify more slowly (slang typically moves faster but also rarely stays in the lexicon long term). Many words are portmanteaus or compounds, like heelback (heel + back). How words are composed is called Morphology[0]. I mentioned in another comment morphemes. Let's look at transgate. We have trans+gate. Trans is a loan word from Latin meanings “across,” “beyond,” “through,” “changing thoroughly,” “transverse". We know what a gate is, but it can also be like a block (gated) or in a circuit (which is like a door). Here the model is taking the morpheme "trans" and using it as if it is "transaction". But in "transaction" the word makes sense because it is through an action (the word started from the meaning to do business and because this often means exchanging money, that's how we now think of it).
So "transgate" also sounds weird because it has opposing ideas. "through" + "block". But we need to look at morphemes to see why. At least (IIRC) it made this word a verb.
[0] https://en.wikipedia.org/wiki/Morphology_(linguistics)