I have questions regarding accounts in the eth network

So I have just learned Ethereum and solidity. Created my first Dapp using truffle and metamask which is great for me at least I have learned the Ethereum fundamentals but a few things are confusing about accounts?

1. When I create an account on metamask, where exactly keys are stored?
2. If the private key is stored locally how does the public key is transmitted to all the nodes to verify my signature?
3. Can two people have the same account address but different mnemonic?
4. How does the account created on Mainnet is same for Ropsten or Rinkeyby?