Afaik, the sequencer node(s) are bundling the transactions and then posting them bundled to Ethereum/Base from Base/B3. Within the timeperiod between a transaction being run on the rollup and it being submitted to Ethereum/Base, a transaction is technically not confirmed, only afterwards. Some blockchains only run 1 sequencer node, which means, there is some trust involved in the sense that the sequencer node isn't censoring transactions or delaying them. However, afaik, there are also projects trying to decentralise the sequencer nodes.
Here is more about Sequencers: https://blog.jarrodwatts.com/the-ultimate-guide-to-sequencers-in-l2-blockchains
thanks