Blockchain A blockchain is an immutable digital ledger that records every single transaction ever made.
Blockchain helps to solve double-spending problem without the need of a trusted authority or central server. Immutable and distributed ledger. Beyond simple transactions, modern blockchains often support smart contracts - self-executing code that automatically enforces agreements when predetermined conditions are met, enabling programmable trustless interactions between parties who don't need to know or trust each other.
Bitcoin : Asset of value that can be interchanged securely without 3rd party. It was designed as a digital alternative to traditional currencies, aiming to provide a decentralized method of transferring value. Bitcoin transactions are verified by a network of nodes through cryptography and recorded on a public ledger called a blockchain. It has digital scarcity (concept of miners).
Proof of Work (for consensus among miners). Computational effort discourages cheating.
Transaction and wallets. All transactions are verified (or rejected) by a global and decentralized network. The corresponding transaction (since the beginning of time) are uniquivocally assigned to my wallet, which establishes its balance now. That's why me, as the owner, do not 'have' the coins. I just have the private key associated to the walleta and to those transactions (and, as a consequence, to the balance). There's a public key associated, which is public and uses to send/receive me money.
BTC is the original Bitcoin's blockchain (1mb per block). Then it was forked to BCH (8mb per block). Then it forked to BTG. All blockchain are still working and valid but they 'trade'/mine separately.