My iPhone 11 screen seems to be plastic instead of glass.

This might be pretty basic stuff you already checked, but you might check to see if you or someone in your family, etc. has a screen protector on it. Additionally, did you remove the sticky plastic strip covering the screen when you first opened your iPhone? (I ask this because many years ago, I used my iPhone 6 Plus with this on the screen for a full three months before I realized it was removable!)

To answer your question, however, no the iPhone screen is not plastic. In fact, it is made out of very durable, high-quality glass that is designed to be resistant to cracking and chipping, and resist scratching. Apple is known for their high-quality displays and that includes the materials they use to make them. Although currently manufactured iPhones (e.g. iPhone 13, 14) use Ceramic Shield screens with 4x better drop performance, the iPhone 11’s screen glass was the toughest in the industry when it first came out.

A word of advice: if you’re concerned that a part of your iPhone is not functioning correctly (or simply isn’t the correct part - it shouldn’t have a plastic screen!) definitely take it to an Apple center and have them look at it, especially if you’ve ever had your iPhone serviced in the past - if it wasn’t an authorized Apple service center they may have used non-genuine parts in your iPhone, which you don’t want anything to do with.

Hope this helps.