1. How staking work for altcoins that are PoS?
You're helping securing your network through staking your token to the network. You will be rewarded from your tokens that already staked in the network.
2. How is your coins used by those who took it as a stack?
Your token just like a power like mining hashrate. This being replaced by token (staking) to make the protocol become much more conservative. This is not used by those but by its blockchain.
3. When you stack, is your coin stays with you or it will be transferred to somewhere else? (the idea of transferring your coins seems to be really stupid)
When you wanna staking your coin and you will be getting a staking address that dedicated to stake your coin in the network. You can withdraw it anytime if there's no vesting period applied to your token.
4. Anything I should read/watch/learn about stacking?
Yes and better to read more about that here
https://coinmarketcap.com/alexandria/glossary/staking