This is really, honestly not hard. Spend a few minutes reading about this, or even better, ask a LLM to explain it to you and clear your misconceptions if regular blog posts don't do it for you. This is one of the concepts that sounds scarier than it is.

edit: To be clear there are tough academic cases where complexity is harder to compute, with weird functions in O(sqrt(n)) or O(log(log(n)) or worse, but most real world code complexity is really easy to tell at glance.