This is mostly a post about rollups on Bitcoin. If you're curious about what it means when someone says something is a bitcoin rollup, it's worth a read.
sats forwarded to @tuma because janusz still doesn't have an account here (that I know of).
Rollups are alternative blockchains that apply their state transition function over data that is posted to bitcoin and made available by bitcoin full nodes.
Bitcoin does not enforce rollup state transitions and rollups do not inherit bitcoin’s double spend resistance. For rollups, bitcoin is the data availability layer that sees rollup nodes to derive their state from bitcoin and gives rollup nodes an ability to enforce finality based on the data posted to bitcoin.
Rollups on bitcoin can be the most trust-minimized sidesystem. They are sovereign systems that use bitcoin for data availability. This enables the rollup full nodes to come to settlement without having to bootstrap their own consensus set, and it lets them build really good bridges if they want to use bitcoin, the asset, on their system.