blockchains are just a fancy tool for everybody to agree on some state of the world and update it
Rollups are blockchains that post their blocks to another blockchain, and inherit the consensus and data availability (DA) of that blockchain.
Ethereum is often referred to as the “settlement layer” for rollups because the L1 smart contract supposedly decides the canonical rollup chain. This is wrong. The bridge provides a view into the rollup. The bridge does not define the rollup. What people really mean by “settlement” here is usually just finalizing an enshrined bridge. That’s also why there’s a messy debate over when a rollup finalizes. People often mistakenly think that an ORU finalizes after a week, but that is just when the bridge finalizes its view. Different “observers” view finality at different times. The bridge is one such “observer” of the rollup (which is slow in this case), but the bridge is not the rollup.
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