I don't understand how the gas fees work

I am trying to transfer some of my cryptocurrency (ex. USDC) into another wallet/marketplace. I believe when I transferred from CoinBase to another wallet, CoinBase themselves paid the gas fees since I don't have ETH in my CoinBase account. The only weird thing is that the amount I transferred wasn't the entire amount and paid a network fee to CoinBase? So, if I "had" ETH in my CoinBase then I could pay for the gas fee myself, wouldn't it have been better for me to done the transfer using my own funds? I am assuming this how things operate in CoinBase (a fee from themselves?) or maybe someone could clarify this part for me. Perhaps I do not understand the difference between network fees and gas fees? Or are there more fees that I need to know of?

There are some questions I have about the gas fees and how they work. First, I am using sites such as gasnow.org, etherscan.io, and ethgasstation.info. When using these sites and trying to figure out the "best" pricing to choose from, I wonder if it's reliable on setting these custom prices compared to using whatever the wallet/marketplace price gives you. The next thing I am confused about is the Gas Limit part. Gas Limit 21,000 seems to be the minimum amount. I am not sure if I should use this amount since I've read that it's only for simple transactions though I don't know what a "simple transaction" is. When transferring a stablecoin such as USDC, is that considered a "simple transaction" or would it fall under a "smart contract"? If so, how do I differentiate transactions that are "simple" and using "smart contracts"? I believe transactions using smart contracts require more fees, thus using the minimum Gas Limit may not succeed. So, how does one determine what Gas Limit to use since I read that using too little would be rejected and using too much would be a waste but you might get refunded for the extra portion of Gas not used? I see that gasnow.org lists the amount of "gas used" under the transaction costs though it brings me back to the same question on how reliable it is to custom set the Gas Limit.

https://preview.redd.it/9ruy8f7pnq171.png?1920&format=png&auto=webp&s=b3a3955abca3643066c5de919934071234112a76

If there are additional resources/better tools to use for these gas fees transactions, it'll be nice to know. These are the current ones I've stumbled upon and I don't mean to be doubtful/bashful on these platforms that are providing good data. I am just a newbie in the crypto space living in the shittest state that doesn't want to deal with crypto making certain exchanges/platforms unaccessible. I am trying to read more before making any big moves/transactions so that I don't get screwed with mismanaging my wallets. Thanks for taking the time to deal with my nonsense.