Well, yes, if you show up at US immigration with no money, that can look a little odd and suggests that you intend to get money while you're in the US. No matter how generous the friends or relations are whom you are visiting in a foreign country, most travellers need some money while they are visiting, if only for emergencies. If you have at least one credit card, that can help. Many travellers pretty much only use a credit card while travelling. The thing about having a job is not that they want you to be employed, but having a job or being enrolled in a school shows that you have ties to your home country and good reasons to go home again, as do things like bills or a house in your name which many young people just don't have yet.
Yes, it's true that they think that people who are visiting girlfriends are more likely to try to stay in the US. They may ask, no matter how you describe the person you are visiting, questions like how long you've known this person and how you know her and frankly, if a man is visiting a woman in another country there's an excellent chance she's a girlfriend (unless she's his sister), no matter what he says she is. there isn't much you can do about what they think. Be truthful, because they can ask anything they want. They trip up people every day who are not being truthful, and it looks worse to be caught lying.