Bitcoin is not only a decentralized digital currency but also it is a protocol for transferring value. Strictly, speaking, there are no any "coins" that are moving inside the blockchain, what is moving there are ownerships over particular UTXOs (unspent transaction outputs). It can be thought of as a transfer of rights for property.

No and no, you get it all wrong. Altcoins may be adding some features bitcoin doesn't have or rather doesn't need to have BUT altcoins DO NOT have features Bitcoin has (for example, fixed supply and decentralization) so altcoins' features are not "extra", they are different at best.

Centralized exchanges are arguably the worst way to get into cryptocurrency, especially if they require users to undergo KYC procedures. The best way, in my view, is to ask your knowledgeable friend to explain to you how it works, also ask him to send you some coin to try it out.