Question: What is THE difference between befall and happen to?
Answer: The difference is this:
'Happen to' is used in modern everyday English - 'befall' is not.
It's normal to say 'What happened to your hair?' or 'I must tell you what happened to my friend' or 'What's going to happen to this building?'.
Nobody says 'befall' unless they're deliberately using archaic language for a specific effect. Forget about 'befall'. Life is short, and you have far better things to do with your time than worry about outdated English words which nobody uses.