Build and Shoot

I started researching servers, vps, coding different things. And I got stuck on VPS. And I can't find the answer. Or I'm not wording it right. So here's my question(s). So I've read that a vps is a basically a section sliced out for your self from a large dedicated server. giving you full access to the server and no one else. I understand this. But What I don't under stand is. Generally you can buy or rent a vps. But I've heard that you can create your own VPS and run it from your computer. A friend wants me to open one and run it 24/7 from my pc. So If a vps is a portion of a large dedicated server cut out for you.
So then how can you make your own. If by definition you are taking server space from someone else?
And if yes. then who's server are you taking from?
Next. Can you have more then 1 vps you created or run yourself? Or are you limited to 1