More importantly, I wanted to know how secure is binance exchange?
Trust =/= security.
Binance is very trusted, and they are known to provide quite good customer service and have a strong presence on social media. These are all why they are so popular.
However, just because they can be trusted doesn't mean that security is necessarily a guarantee. We don't know exactly how they handle their cold storage (although I'd assume that they probably do a better job than some of the rest, since they haven't had any major hacks or security issues over the past few years of existence), nor how well they will uphold their standards in the future.