Me vs Mine: Show Possession in English - Basic English Grammar

In this video, Alisha answers 5 questions. - What's the diffrence between 'switch on/off' and 'turn on/off'? What's more casual? - I hear americans pronounce the article "a" before a word -in a sentence - with the sound "eɪ" and sometimes pronounced with the sound "Ə", is there a rule about that. - What is the difference of using "...,yet ..." and "..,instead of ..." and "...,despite..."? - Is there any difference between "my" and "mine" (eg. ""he is my friend"" and ""he is friend of mine""). In what situation should I rather use my/mine. Can you explain please?