My I-94 is expiring on 23rd Dec but I applied for visa extension on 5th Dec. I spoke to an immigration lawyer and she told me I can stay as long as my decision is pending. If the decision is Yes, then all good, otherwise I'll have to leave immediately. Is that how it works? I'm on a B1 visa.
Yes, if you have timely filed (i.e. USCIS received it before your status expired) a non-frivolous extension of status application, you can stay in the US for as long as it is pending, regardless of whether your status has expired or not. It currently takes a few months for them to give a decision on the application.