1.) It's a pretty big spectrum as far as the US is concerned. Some people have sex planning on never seeing the person again, some people wait months or more until having sex in a relationship. Sussing out this information in a way that doesn't put people off is, I think, one of the parts of the early stages of dating/hookups.

2.) One of the things that has been pointed out to me by non-US citizens is the idea of a 'friend gap' in the US. Most people in the States have a lot of casual friends and acquaintances, but very few close friends. Relationship advice is fine, but overtly sexual relationship advice tends to be a close friends only thing. If they're sharing details of their sexual relationships with you, odds are they're a close enough friend that you can give them advice on it, as long as you don't deliver your advice in a cruel way.

3.) The 'friend zone' mostly comes from an understandable mix up between men and women in America. Women in america tend to have more emotionally intimate friendships across the board, where emotional intimacy among men is sadly seen as weak. One of the few relationships in which heterosexual men in the United States are allowed to be emotionally intimate is in a romantic or sexual relationship. Because of this, a male-female friendship can evolve to a level of emotional intimacy where the man thinks "This is evolving into a romantic relationship, we should be dating" and the woman thinks "We're just good friends."

Building on that, most people are at least somewhat risk-averse when it comes to dating someone. Most of the men in this relationship enjoy the level of intimacy there is already, even though they would like something that has the ability to progress further, whether that's sexually, romantically, or otherwise. Most of the women in this relationship grow increasingly risk-averse about losing that friendship and if they're unaware of romantic advances then it becomes this uncomfortable, gaslight-y sort of revelation that undermines the friendship if the guy reveals his desires. Weirdly, the friend zone happens at least partially because you end up upside down in this sort of friendship-mortgage where the friendship is too valuable for both parties for either of them to jeopardize.

There are, of course, people who abuse this social speedbump. There are men who will build a friendship under the assumption that they can transition casual favors and being a constant emotional whipping boy into a sexual relationship, and who get really indigent when that doesn't work. There are also women who maintain friendships with these toadies knowingly, and use them as something between a butler and a fallback boyfriend in case the type-A personality people they usually date pass them by. Both of these are shitty, but the lady here is honestly less shitty in my mind, if only because manipulating an idiot who's trying to be socially manipulative seems like fair payback.