I need to know about _UILayoutGuide, like what it is, what it does and why it present in the hierarchy of UIView as a subview with almost always frame = (0,0,0,0).
UILayoutGuide is normally referred to -topLayoutGuide and -bottomLayoutGuide, those are not really constraints, but they are view elements conform to a protocol called UILayoutSupport.
You can find more info about that protocol here. The value is often 0 but you should pay a lot of attention where you ask their size.