Collecting all my different addresses in a spreadsheet, I realized that on the exchanges "Huobi" and "Gate.io" my BTC Wallet Adresses were identical to my USDT Wallet Adresses on these exchanges. ...

Why do USDT and BTC have the same receiver address?

Introducing USDT
update at 2019-06-14

Basic information:

• Pegged currency: US Dollar
• Issue date: 2014
• Company: Tether Limited
• Market value: about $2.2 billion (as of 2019.3)

There are many types of USDT released by Tether, two common ones:

Bitcoin-based USDT
This type of USDT token is based on the Bitcoin network's Omni protocol. The receiving address is the same as the Bitcoin main address. The transfer transaction requires Bitcoin to be used as miner's fee. More about Omni protocol.

Note: Do not use Bitcoin child address for USDT receipts.

Ethereum-based USDT
This type of USDT token is based on the Ethereum ERC-20 standard. The receiving address is the same as the Ethereum address. The transfer transaction requires Ethereum to be used as miner's fee.