The concept is that by aggregating things we observe that form parts of processes, or physical things, sometimes we end up calling them one name, or feeling them as one thing as a category or a concept.

For example, if we see a sequence of events where the first gave way to the second, which gave way to the third, fourth, etc. all the way to the 10th, because each one was not possible unless the previous one had happened we tend to "feel" as if something is passing, so we feel time, and we call it time, and create methods for measurement, and it all works in favor of our collective organizational strategy, but time by itself does not exist. The use of the word "cognitive illusion" I got it from some authors who describe it this way.

In my case I had arrived to the conclusion that time didn't exist, that it was only "movement", independently when I was in 10th grade, when a history teacher asked aloud "what would happen if everything in the universe stopped?" and I screamed "time would stop!" and she sent me to detention for acting as a smarty pants. I learned about relativity much later in my life.

Regarding free will, I had the same belief as you, but they convinced me otherwise here, when I posted about my idea of free will, randomness,and determinism.

Money, because I am in finance, I have a more closer perspective and have considered it similar to time. The addition of the properties of store of value, unit of account, medium of exchange, divisibility, portability, etc. add up to a feeling of substance or tangibility. Just like you say "I have checked banknotes in my wallet; they are real..." that is what I call above an illusion. That paper you have is just nearly worthless, but because we use it as a standard it has a subjective value and that feels like "money".