user124492: My goal is to add a new feature to Bitcoin, which requires changing the structure of the Bitcoin blockchain. So I was wondering if Bitcoin Core supports having a new/own Blockchain? If so, how to do it? Right now, I manage to run bitcoind but it seems to sync from the existing Bitcoin blockchain.

user47827: Bitcoin Core supports creating a custom signet and mining utility for setting up a public test network with alternative functionality. This is not meant to be used to create an altcoin with actual or perceived monetary value as I (and many others) would recommend using a sidechain rather than attempting to start a new currency.

Right now, I manage to run bitcoind but it seems to sync from the existing Bitcoin blockchain.

Yes running bitcoind is for mainnet and bitcoind -testnet and bitcoind -signet are for testnet and the default signet (not custom signets).

124492: <p>My goal is to add a new feature to Bitcoin, which requires changing the structure of the Bitcoin blockchain. So I was wondering if Bitcoin Core supports having a new/own Blockchain? If so, how to do it? Right now, I manage to run bitcoind but it seems to sync from the existing Bitcoin blockchain.</p>
 47827: <p>Bitcoin Core supports creating a <a href="https://github.com/bitcoin/bitcoin/tree/master/contrib/signet" rel="nofollow noreferrer">custom signet</a> and <a href="https://github.com/bitcoin/bitcoin/pull/19937" rel="nofollow noreferrer">mining utility</a> for setting up a public test network with alternative functionality. This is not meant to be used to create an altcoin with actual or perceived monetary value as I (and many others) would recommend using a sidechain rather than attempting to start a new currency.</p>
<blockquote>
<p>Right now, I manage to run bitcoind but it seems to sync from the existing Bitcoin blockchain.</p>
</blockquote>
<p>Yes running bitcoind is for mainnet and <code>bitcoind -testnet</code> and <code>bitcoind -signet</code> are for testnet and the default signet (not custom signets).</p>
