I think you can use "I graduated" when you want to refer to the specific act of having graduated at a certain time or from a specific college. For example "I graduated three years ago" or "I graduated from the University of Technology". On the other hand, "I've graduated" would be used to focus more on the fact that you HAVE done it (instead of not having done it). Example: "I have graduated from college, finally!" You could see it clearer in the form of questions: "Have you graduated?" vs "Did you graduate?" I could go on and on lol. Hope it helps! :)

Thank you so much, you were very helpful

You could use either Present Perfect or Past Simple. If you've graduated recently, maybe within a year, you should use the Present Perfect. If it's been more than a year, Past Simple is better.