Query for entering first time travelling to USA carrying H1B visa

I am a first-time H1B visa holder, I am currently residing in India and due to the Covid-19 restriction, I am not able to plan my travel. However, I heard that
people are taking the route of the third countries like Serbia and Mexico and reaching the final destination US. Could you please help me if I will take the route of Serbia or Mexico, I will be able to enter the US border?

Furthermore, I want to know the Immigration officer will stop me at the port of entry and ask me why I am traveling during pandemic time cause I am not carrying an NIE certificate.

Thanks in advance!