***** NOT A TEACHER *****
Good afternoon, shwetank.
(1) I have never heard of the second one.
(2) Preaching to the choir = an idiom that means you speak to
a group of people because you want them to do something or
believe in something, but they ALREADY agree with your ideas.
(a) Let's say that you belong to political party X.
You talk to a meeting where everyone else also belongs to political party X.
So, of course, everyone at the meeting will applaud everything you say.
They will cheer everything you say.
In other words, you are preaching to the choir.
Have a nice day!