How do we obtain the computer's IP with whom we want to communicate ?

Hi , so I'm currently learning network in my curriculum and the professor said that if you want to send data with a PC(A) to a PC(B) , the data will be 'cut' in little packet and these packet have a part named "control information" which need to have the PC(B) 's IP to be send correctly.

My question is how do we obtain PC(B)'s IP given that PC(B)'s can't send his IP to PC(A) since PC(B) doesn't know PC(A)'s IP too.

I asked the professor about this and he said something along the line of "well it happens" and made an analogy with when you want to communicate with someone using your smartphone you need to have his portable number but that didn't really help me.

Sorry if it's not an intelligent question .