кто они(по профессии/виду деятельности)

And note that “they“ is definite (unlike “those”). In other words, the things in question have already been mentioned and are “on the table,” in the sense that you both know what you’re referring to.This is not an opening question in a conversation. For example, let’s say that your friend comes back from China and wants to show you something he brought back. He opens a box filled with some strange little things and asks “What do you think of these?” you can answer “That depends – what are they?“ This is because you’re both looking at the same thing, so you use the definite form “they.“ However, let’s imagine now that you walk into your friend’s room, see the box open on his desk and ask “What are those?“  Here, you can’t use “they“ because you’re referring to them for the first time and your friend won’t know what you’re talking about until he looks over.