A user’s transaction on Coinbase rarely represents an actual transaction on the blockchain. A transaction actually represents a row in a Coinbase SQL database, which Coinbase will eventually batch and then convert into a blockchain operation.
But something like Uniswap: https://app.uniswap.org/#/swap is fully on-chain. Every trade happens on chain, because the application is an immutable smart contract liquidity protocol, called an AMM, or automated market maker.
It does as much, if not more, volume than Coinbase ($1.98B in last 24H): https://info.uniswap.org/#/
False. MetaMask uses Infura as a default, but you can edit your RPC URL to whatever you want. You can even run your own node and sync the chain if you've got a couple hundred gigabytes to spare (no stake required to run a non-validating node).
They don't have to hit the blockchain all the time. Only when it makes sense. For everything else, there's off-chain signatures. That's the beauty of it. Hey, you don't want to use the chain, just use the sign-in with Ethereum feature, and that's totally cool!