There is not only one blockchain, but multiple blockchains. Bitcoin is a blockchain. Ethereum is a blockchain; Neo is a blockchain. Each ledger is a blockchain with its own network and mechanism. The same can have several blockchains: one main for production and others for testing purposes. In principle, there is no relationship between them.
Some blockchains are developed with a protocol that allows the creation of bridges to other chains, but this is the exception and not the rule. So far, interoperability between blockchains is low. There are projects to create blockchain networks, but their use is not widespread. It is possible that a long time will pass with several protocols competing with each other for marketshare, without a protocol being able to dominate over all the others. The future is multi-chain.
The philosophy behind blockchain technology has also broadened a lot since the arrival of Bitcoin. Ironically, despite having its beginnings as a movement against the government, banks and large corporations, nowadays these institutions are including the use of blockchains in their technology stack. Such enterprise blockchains have an ingredient that makes them totally different from the first blockchains: they are private or permissioned, which means you need permission to be a participant in the network. Nothing is further from the idea of a fully and decentralized network.
Many ingredients make blockchains different from each other. Some protocols implement only minor modifications, while others implement completely new functionalities. As an example, Litecoin is a blockchain protocol very similar to Bitcoin, and it basically changes only the maximum size of its blocks. Other protocols have developed new consensus mechanisms to improve network scalability. Since such protocols are open source, any developer can make a copy and start their own network. This is how the ecosystem has evolved.
These days, blockchains are everywhere. So if you hear the phrase - it’s on the blockchain - the first thing you need to ask is: “which blockchain?“. During this course, we are going to study different protocols of several networks. We are going to see what are their similarities and differences. Maybe, by the end of this course, you might not want to start your own network.