1. Can blockchain be implemented without cryptocurrency?
Yes. Blockchain is a way of storing data, where each block is chained to the previous one. For example:
Block 1:
{ data: 'foo', previousHash: '0000000000000000000000000000000000000000000000000000000000000000' };
Block 2:
{ data: 'bar', previousHash: 'd807da8ba0473afd6cb13d465d21e2d1e0e14598d8555afe5fe3e88c7e114b05' };
where the previous hash in "Block 2" is the "Block 1" hash.
2. Are apps built on Ganache centralized or decentralized ?
Ganache is a local blockchain and Ethereum-like network emulator usually used for development and testing. Multiple instances of Ganache are not able to communicate with each other, so by this definition, it's a centralized system.
However, apps that are built and tested locally on Ganache, can be deployed to decentralized networks such as Ethereum Ropsten testnet, Ethereum Mainnet, BSC Mainnet, etc.
3. Does every smart contract, Decentralized application, Ethereum Transaction, all of it live on One Giant Blockchain, the Ethereum Main Network ?
Smart contracts and transactions - yes, they all are stored in the Ethereum network blockchain.
DApps - This consisted of smart contracts and off-chain apps (usually written in JavaScript and other web languages) that are usually hosted on a private server or a regular shared hosting.
How is that manageable
The Ethereum blockchain is currently between 0.5 TB and 1 TB, depending on what node software you're using, and it's growing over time (source).