The exam only concentrates on GET and POST seeing as in real life they are the only two methods you are likely to use. However, you may be asked questions such as which method is useful for requesting only the header and so you only need to know the nasic function of the other methods.
As regards idempotency you need to know that GET can cause no side effect (i.e is idempotent) while POST can cause side-effects (not idempotent)
Remember idempotency is good. Only with POST do you need to ensure that orders are not sent twice etc.