This is usdt source code from tron.
tron
https://tronscan.org/#/token20/TR7NHqjeKQxGTCi8q8ZY4pL8otSzgjLj6t/code
ethereum
https://etherscan.io/token/0xdac17f958d2ee523a2206206994597c13d831ec7#code
USDT相关问题：
- 在以太坊版本中transfer没有返回bool https://learnblockchain.cn/article/506
- 在tron版本中返回transfer为false问题 https://solidity123.com/solidity-error-survey/tron-usdt-safetransfer.html