What is "Magenta"?

Hi, just a quick one. This is my first time with a phone number in the USA. I have an e-sim. When getting the e-sim, I spoke to an agent on phone who did everything! She reaffirmed that I was getting a prepaid service after I asked (to make sure) various times! She said with the Magenta "plan" I was getting this and that! I asked again: is it a plan or is it prepaid? She said it was prepaid! So my question is , I am getting notifications that my "plan" payment is due in a few days... is Magenta a prepaid service? Or can I pay when I have the money?

Thank you beforehand!!