Not every crypto project of the thousands that are listed on coinmarketcap or coingecko or whatever has it's own blockchain. Bitcoin of course is has it's own blockchain and obviously it was also the very first blockchain because Bitcoin invented the whole cryptocurrency thing. Ethereum also has it's own blockchain and that is not only a "normal" blockchain like the one from Bitcoin because Ethereum offers smart contract functionality which means that you can use Ethereum to make Dexes, to programm Defi applications and also to create tokens. Which means that a lot of the crypto projects, even big ones are just tokens that were created on the Ethereum blockchain or others ones.