Not all altcoins use the same blockchain as Ethereum. While some altcoins are built on the Ethereum blockchain, many others use different blockchain technologies.
Ethereum's blockchain is based on the Proof of Work (PoW) consensus algorithm, which is also used by some other altcoins. However, many altcoins use different consensus algorithms, such as Proof of Stake (PoS), Delegated Proof of Stake (DPoS), or Byzantine Fault Tolerance (BFT).
For example, Ethereum's blockchain uses the Ethash algorithm, which is a modified version of the Proof of Work (PoW) algorithm. However, other altcoins like EOS and Tezos use the Delegated Proof of Stake (DPoS) algorithm, while Cardano uses a proof-of-stake (PoS) algorithm called Ouroboros.
In addition, some altcoins use different types of blockchain networks, such as private blockchains or federated blockchains, which are not open to the public like the Ethereum blockchain.
Therefore, while some altcoins may use the same blockchain technology as Ethereum, not all altcoins do. Each altcoin has its unique features, consensus algorithms, and blockchain networks, which set them apart from one another.