Help : How can one token/coin be present in many chains?

Sorry might be a noob questions, but i am very confused!

1. How can one token/coin be present in many chains?
   e.g., USDT or Maker is an ERC20 token; so when I try to withdraw the same I see option of network like ERC20, BEP20, TRC20, Polygon, Solana etc.,
   so lets say when I buy the token say USDT on a exchange like gate.io or binance.us, which network does it buy? and when I transfer it to lets say a wallet, what happens to the token on original network?
2. Metamask -> so when I add different networks in metamask like fantom, harmony, BSC etc., they all share same address, does that mean all of these are erc20 as well or not? how can they same address?
3. based on above if I need to transfer USDC from gate.io or binance.us (now supports bep20 withdrawal) to metamask BNB address, should I just choose bep20 on withdrawal side and give address from metamask from BNB network?