Without actually looking into it... Probably the same way that "Xapo wallet to Xapo wallet" or "Coinbase wallet to Coinbase wallet" transactions are free... That is to say, they are not actually blockchain transactions.
Instead, they are just the service provider moving some numbers around in their database. All of these services seem to work on the fact that you don't actually have control of the private keys... so the coins in your wallet are really just a number in a database. When you want to send some coins from your wallet to your friends wallet on the same service, they just deduct the number from your account balance and add it to theirs. It does not require a transaction to be broadcast or confirmed by a miner... so no transaction fees are required.
Thank you!
Now i couldn't understand it at all
When I going to Bitfinex or Bittrex I am able to generate my own address to Transfer Tethers..So, this is not database only..
