If lost, how to retrieve the BTC?
It will not disappear, because the BTC wallet format will not respond if you enter the ETH wallet address. When you enter the ETH wallet address and you send it will say 'Your wallet address is wrong'.
Unless you send BTC to the BTC address, but there are one or two numbers that are wrong on the BTC address, then BTC will be sent, and it will be lost, then it cannot be returned to the base wallet.
So don't think about sending BTC to ETH address, then BTC will change to ETH, or vice versa. Sending a token (erc20) to an ETH address will also not change to ETH. They have their own code.
But if you send an ERC20 token to any ETH address, the transaction process is successful. But it cannot be returned, because the ETH address does not have a key.