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.