In your first transaction, you just set it to private so only you and the recipient (and Venmo and whatever gov't agency) can only see the memo line. The amount is always private. The setting sticks after that so you don't ever need to change it again.
No fucking clue. Some people like to leave jokey memo lines but I'm glad none of my friends have done something like that outside of the "friends only" option. I just say what it's for or give it a generic name like "money" or "here you go".