Who decide each transaction cost how many Gas?

Correct me if I’m wrong. Based on my understandings of gas price:

1. GWEI (unit of gas): is fixed 0.000000001 ETH no matter what

2. Gas price vary and it’s usually 40 GWEI to 2000 GWEI depending on time, demand, transactor’s bid, the verification speed they want.

3. A basic standard ERC20 transfer cost 21000 Gas. Other smart contracts might be cost more. My question is who decide how many Gas each transaction cost? Is there a fixed amount set for each category of transaction? Or the number of Gas also varies for each transaction? If so, I hear it’s the user who can submit the gas limit and system would return the unused limit back to your wallet. But when I registered for OpenSea and verified a smart contract when it used to confirm my user account, my MetaMask wallet didn’t provide my the option to choose how many gas I want to spend, nor did it show me the speed I can choose from. It just show me the value of ETH (the gas price) for this transaction and I can only choose to accept it or not.

So I don’t understand :1. Who decide how many Gas it cost for this transaction? Is it OpenSea or it’s a prefixed number for each type of transaction in Ethereum network, or the market itself? 2. In which scenario of transactions can I have the option to manually confirm the speed regards to how many GWEI I accept? Or in my case, is OpenSea automatically choose the fastest speed for all its users?

Thanks for any comments that may help!!