How can someone be ethnically Jewish?

I was having a debate, which I won’t get into the topic on here cos that’s not what I want to do – I want to be educated. But I was having a debate and someone brought up that Jew was an ethnicity. I was so confused? I said “You can’t be ethnically Jewish?” “Like I’m from a family of quakers?? But I wouldn’t call myself ethnically Quaker?”

And I genuinely thought that was not possible and doubled down. Anyways I was called all sorts of names, bigot, antisemitic – truth is I genuinely had no idea. I’m clearly very uneducated to Judaism and Jewish people in a way of – I have no idea, not that I have prejudice towards you folk at all. So please educate me, I’m not a bigot as I’ve said, just ignorant and uneducated clearly and unfortunately I can’t take back my statements but can learn for next time.