To my mind, ‘good to see you’ is something you say only to someone you already know. When meeting someone for the first time, ‘good to meet you’ feels more appropriate to me, perhaps because it puts more emphasis on the meeting. But increasingly people seem to be saying ‘good to see you’ even when they’ve never met you before.
It’s a similar story with ‘How are you?’ To my ear, this is only appropriate for someone you already know. If you don’t know someone, ‘How do you do?’ is better. But nowadays, people you don’t know are very likely to say ‘How are you?’ To me, this feels slightly intrusive.
‘How are you today?’ seems to be particularly common in service settings, for example in shops or restaurants, or if someone is cold-calling you. I suspect sales people invented this as a strategy for building rapport quickly by faking intimacy.
I know these are all only conventions and don’t matter much. But this dinosaur won’t be using the new ones for a few more years.