These are known as Direct to Film or DTF (lol). They are quite literally a printed plastic sheet that’s melted to the shirt. Easiest way to check for this is to crease the shirt on the design. Screenprinted will reveal the shirt color underneath where DTF will be a solid sheet.

I hate those

Those use to be pretty popular when I was growing up, I hated them. So much so that I barely buy t-shirts these days unless its a free shirt from some company

They also create weak points along their edge since it needs to bend more to make up for the stiffness of the plastic part. It'll start coming apart there before the neck frays.