Depends on the interface/device you're using, but most common way is to tap the screen, then tap more, and then tap raise hand. Or, you can do like Chris and just holler out that you want to share something. If you have something to add to an ongoing conversation, unmute yourself and chime in when the opportunity arises. Sometimes you have to just jump in, but I try to remember to give people a chance to chime in before moving to another topic/person. It's not easy to do it all, though, and sometimes I miss someone who is trying to get a word in edgewise.
Of course, you have to be on the zoom chat, not on YouTube.