What’s the hard thing inside my (23F) boyfriend’s (23M) ass?

Okay, so I had my first sexual experience with pleasuring a man anally a few days ago. I had my finger up his ass and a vibrator vibrating my finger as well as jacking him off and he loved it. I loved it because I get off on getting him off. But my question is, what’s the hard thing in his ass? Is that the prostate? And is that what I’m supposed to be stimulating when I’m up there?

I basically just want to get better at it. And it’s just a general curiosity because I wasn’t expecting anything solid to be up there. And it wasn’t poo… because it wasn’t squishy.

Any advice or information is welcome! Thank you!