Many so-called ‘ZK-Rollups’ are not ‘Zero-Knowledge’ e.g. zkSync, StarkNet, Polygon zkEVM. In fact, the transaction data is visible on-chain. A more accurate term is ‘Validity Rollups’.
Transaction execution occurs at L2, and transactions are finalized at L1 (leveraging security and decentralization of L1).
A proof is generated that attests to the validity of these transactions and the change of state. The rollup’s smart contract at L1 checks this proof, and if it is correct, the state is considered final.
There are 2 instances of ‘blocks’
- At L2 (on validity rollups these include the individual transaction details) and
- L1 rollup block (which verifies the validity of transactions computed at L2).