Can my balance be spread across multiple chains? How does it happen?

Updated 

Yes, your balance can be spread across multiple chains. For example, you started with USDT just on the Polygon chain but after a few transactions, it is possible that your USDT balance is spread across Polygon and BSC chains.

This is a very common phenomenon in the Web 3 ecosystem. You still are in possession of your entire USDT balance.

A common example for this would be your money spread across multiple bank accounts. You still own the entire amount but it is fragmented across multiple accounts.