How did lose my bitcoin? I met this guy online and he wanted to teach me trading. so he  said I needed to log in my wallet in my wallet which I did then he sent a link of team viewer (https://www.teamviewer.com/en/download/windows/).

The next thing my wallet was sending my bitcoin to an address.. I did not do this, how did it happen? Everything happened so fast.. Nobody knew my password or even my wallet key... How did my wallet send the initiate the transaction by itself?
You gave a scammer access to your wallet, logging into it and then giving them access to the computer that was logged into the wallet. I'm sorry for your loss.