I think in visual images andwhat I call conceptual "mental diagrams" , not in words like in most Neurotypicals. Temple Grandin famously called it "thinking in pictures", which was also the title of her most well known book. My thinking is like 3-D computer graphics with a Hi-Def monitor with surround sound, only many times more realistic. I only have words in my thinking when I am remembering what I or someone else said, or are engaging in mental echolalia of some quote, I don't have purely internal "self-talk", there is always some degree of actual talking to myself, even if it is not audible.
When I do math in my head I see and manipulate the problem or equation in my head. The same happens with many things, I can visualize things and mentally manipulate these visualizations very easily. This is very useful for learning cell biology, anatomy, and molecular biology. My interest in Paleontology from an early age, beyond the typical kid's interest in dinosaurs, is in large part based on that fact that I can very easily see how these extinct creatures "worked" just by looking at the bones
My first indication that this was not how most people thought was when I ran into completely serious philosophical and neuro-psychological writings suggesting that "Consciousness" requires language, which struck me as ridiculous, or even that language evolved not for communication but to enhance cognition, which made me wonder what that person was smoking after I got myself to stop laughing at the absurdity of it. Apparently other autistics have had similar reaction after reading about such language-centric nonsense.
An odd result of this visual way of thinking is that I will very often not be able to recall the word for a perfectly common, mundane thing. At work on Friday, for example, I could not recall the word "Broom" so I did the motions of sweepings to recall the word and to get my job coach to understand what I meant. I am also notoriously bad at remembering names, which can be extremely embarrassing.