I am a little new in crypto world and want to know what are exactly these ERC-20 tokens? How they came into existence and why every token is ethereum based why not bitcoin or another, as far as I came to know.
Just like the previous person suggested, get started with that link posted. In my own understanding, ERC 20 tokens are based on Ethereum blockchain owing to the convinent associated with the blockchain, there are other ERC tokens but most projects tends to go for ERC 20. Also, like earlier stated they are based on Ethereum blockchain hence their existence. There are other blockchains out there such as EOS, XLM etc but every project knows what they want to achieve therefore choosing the blockchain which will serve them. Also, most projects only launch on Ethereum blockchain to kick-start their platform while having a specific time to migrate to their own blockchain, projects with this idea have it on their roadmap as "mainnet". Also, most projects might launch on Ethereum blockchain first then alone the line might find a blockchain which they believe will serve better thus migrating to it. So every project choose a blockchain according to their objectives.