I still don’t understand what it means when people say stuff like “ChatGPT just predicts the most likely next word with the highest probability” or “ChatGPT is

I still don’t understand what it means when people say stuff like “ChatGPT just predicts the most likely next word with the highest probability” or “ChatGPT is just a probability machine”. Concretely, given the N most recent words, what algorithm are you proposing/claiming it uses to assign probabilities to word N+1? Just saying “it chooses the next word with highest probability” doesn’t explain how the probabilities are estimated.