As someone who both experienced phases in life where no one approached me and phases were I get approached regularly, it's a mix of external signifiers and some internal woo stuff that people don't really understand conciously. Or said another way, when someone says you have to "look approachable" what they actually mean is that a) you have to present yourself externally in a way that makes people more likely to engage you (the aforementioned hair, clothes etc.) and b) you have to internally be open to the world (which is what dictates your body language in subtle ways that apparently get picked up). The issue is when someone says something like "have an open body language" is that it's impossible to 24/7 fake a certain type of body language, you actually have to believe it.

If you are naturally a distrusting person people will pick up on it, just how people will pick up if you're naturallly an open person. (The true trick is realizing that "naturally" can be changed)