The blockchain that used by bitcoin was different than altcoin. Some altcoins have similar blockchains, i meant the blockchain that compatible from one to the another blockchain. Ethereum has its first layer blockchains and there are some altcoins that develop the layer 2 and 3 blockchains that compatible with ethereum. Mostly of altcoins were building as assets that used blockchains that owned by the coin. You must know the difference between the token and coin.
Some altcoins have their own type of blockchain.