If you say that something is supposed to happen, you mean that it is planned or expected. Sometimes this use suggests that the thing does not really happen in this way.

If something was supposed to happen, it was planned or intended to happen, but did not in fact happen.

If you say that something is supposed to be true, you mean that people say it is true but you do not know for certain that it is true.

You can use 'be supposed to' to express annoyance at someone's ideas, or because something is not happening in the proper way.

You can use supposed to suggest that something that people talk about or believe in may not in fact exist, happen, or be as it is described.