Question from reddit
How can Arbitrum or Optimism update their rollup with an admin key? I thought SC were immutable?
Answer from whalesniper
The code of a SC is immutable, but immutable code can still upgrade itself, with admin help. It's not that much of a mystery.
Answer from daniel hines
The code of smart contracts is indeed immutable; however, because they are essentially Turing-complete, there are many ways to modify their behavior. Ethereum's guide to upgrading smart contracts is a good primer.
It's worth noting that upgrading the smart contract bridge in e.g. Arbitrum is controlled by a multisig, and can update immediately with out warning. This part is the enshrined part on Tezos, and controlled by the usual governance process.
Response from Nomadic Labs
The SC code is indeed immutable but there are several layers of code interpretation in the rollup infrastructure: the EVM machine that is emulated in the rollup can be updated. Again, this depends on the configuration of the individual rollup.