If I am thinking about how to express a complex idea in writing or speech, I’ll certainly have an “internal monologue.”

But if I’m just thinking about something myself, my thoughts just exist as abstract thoughts that aren’t tied to language. My brain is constantly thinking, but I’m not thinking in English!

I could express the same sort of disbelief you did - I can’t imagine how slow and inefficient it must be to have to do all your thinking in terms of words as opposed just operating on the underlying ideas directly.