Dreams are one of the mysteries we human still have to figure out and understand. Not there yet and may never understand. Till than.
They can come true. As it has been for countless people. Snakes in dream isn't considered good. Are you surrounded by good people? Like, you trust them?
Yes, and like your large snake we dream about people know/loved/lost/ betrayed us or fears we have, and it is natural to help us cope subconsciously as we sleep.
Dreams that happen well we sleep? Those are our imagination creating things for us to see, some for a reason usually created by the subconscious. Others just a culmination of events blended together.
The dreams we have well awake and aware tend to be our goals that aren't subliminal "I want 'x' to happen but first I need 'y' then 'a' then 'z1' to achieve that"