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.
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).