Will nikaah takes place if a boy asks a girl “will you marry me”?

What’s the maslah pertaining to the question “will you marry me”?

I learnt a while ago that whether it was done in a school play for example and the boy had to ask the girl or vice versa “will you marry me” and she said yes, they are married? What would be the maslah and how would the marriage be terminated?

What would happen if it happened on WhatsApp or in private? Would they still be married? And how would the marriage be terminated?

What would be the maslah if the people don’t know about the maslah of getting married in this way and they did it? I have a friend who went out with a group of friends and this girl asked him will you marry me and he said ya I will and they were with other friends? Are they married and what would be the next step?