In the UK we say 'Hello' or 'Hi' and also 'Good Morning' or 'Good evening'. These last two are slightly more formal, but I say 'Morning honey' to my wife. We do not say 'Howdy' in the UK.

Great question. It depends on the person and place. For example I might say "Yo", "Hey", or Hey, What's up. Or my coworker of 10 years, who is like my sibling, I might ask them "What are you buying me for lunch today?" lol

I always go with “howdy”.

Fir someone you have known for a long time it depends on your relationship. Examples: Hey, mom... You do not have to use a greeting in all situations you can just say their name and begin talking if you are really close. Hello, sir... Hello is polite, so it is directed towards those who have a superiority over you. Such as a boss or teacher. Hi, is a quick greeting to someone you are well acquainted with. Howdy, is for those who you meet in a casual conversation.

I would probably respond with a "Hey" or "What's up" or evene a straight out "HEY! how have you been?"