So if person A has USDT on Ethereum and person B on Tron, basically those are 2 different tokens a d person A cannot send Tether to person B?
Yep, correct. Since both blockchains are isolated on its own network, inter-transaction of USDT are not possible.
Another scenario is A send USDT on BEP20 chain to B, but B is expecting to receive it on the ERC20 chain. Due to the nature of the underlying technicalities, the transaction will be accepted but the transaction is only happened on the BEP20 chain, not on ERC20, thus B did not receive the token.
That may be the simplest explanation you could get. But just for example reason I would share mine.
Different chains, different roads or lines.
For example: AT&T can only call AT&T subscirbers. Verizon does the same. So both companies cannot just jump on the lines thus, they will need a bridge for their subscribers to call with each other. I think what he is looking for is the bridge.
An exchange could help at this kind of situations since I don't have a clue if they ever made a bridge that would connect two chains yet.