A sidechain is initialized by an SPO, or ‘chain creator’ who publishes the sidechain Plutus contract implementations including committee registration scripts. These registration scripts allow other SPOs to register or de-register as a candidate in the committee. The SPO also runs a sidechain validator node which starts the sidechain when a quorum of validators is reached.
The sidechain is validated by an SPO committee where each SPO is a candidate running a sidechain validator node. In each epoch, a committee is randomly selected and handed over to the next.