While visiting the chat section I found a room named Backup Room – The h Bar.Though it seems to me that people don't chat over there rather I found that list of hot network questions are posted. But who post these? Is it automatic? Why is it greyish rather that blue in color?
That room is the chat room associated with Physics Meta, in the same way that the HBar is the chat room associated with Physics. That's probably why the color scheme is different.
There was a period where the HBar was one of the most active chat rooms on the SE network, and it wasn't uncommon for two independent conversations to develop at the same time with enough activity that it was confusing about who as talking to whom. During those times the participants in one conversation might decide to move their topic to the "backup room" to make things easier to follow. That was most common during our weekly scheduled chat sessions, but it happened other times, too.
Around the time that I was elected moderator, the HBar needed a lot of moderator attention, and we started using HBar and the "backup room" in a way similar to the divide between Physics and Physics Meta. In HBar people talked about physics, and cooking, and politics, and life; if someone's feelings were hurt and there needed to be a conversation about what kinds of things were or weren't okay to say in chat, whether it had been appropriate to flag or delete a message from someone, and so on, we would have that meta-conversation in the meta/backup room so that main chat could get back to physics and cooking and life.
As our community's understanding of itself matured, we needed the backup room less and less often. Sometimes a user would post an HBar-like question in the backup room and no one else would reply for a week or more. When a user wrote a bot to identify new Hot Network Questions (only a few months before changes to the SE API made that information available in a more robust way), we decided to allow the bot to post in the backup room because it existed but didn't get much traffic. Since the HNQ bot now posts in there a few times a day, I no longer know whether new users still try and fail to chat there. We haven't needed it for meta/moderation purposes for quite a while, for which I'm glad.
I suppose the current state of things is pretty confusing-looking for a new user who doesn't have that history. If anyone has a suggestion about a change to make so that the Physics Chat ecosystem makes more sense to new users, they ought to propose their change in a new Meta question and link it from this one. But I'm happy if it's just a curiosity.