Who controls delegation to new steemit users?

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The delegation is not part of the blockchain code, but the steemit website/API. What's in the blockchain, is the support for operations to create new accounts and delegate vesting shares. This can be used by any user to create new accounts or delegate SP, not only @steem.

The Steemit account signup is handled by a blockchain interface tool called "faucet" (https://github.com/steemit/faucet), which is integrated into "Condenser" (https://github.com/steemit/condenser), the steemit.com website. Whenever a new user signs up via steemit.com, the "faucet" tool interacts with the Steemit API to create the new account via the @steem account on the blockchain and triggers some initial SP delegation to the new account. The delegations are typically updated with "redeemer" (https://github.com/steemit/redeemer) as soon as the new account earns own SP or is inactive for a while.